Tutorial: Basic LEGO Bricks Cake

For my son’s sixth birthday, I threw caution to the wind an attempted a LEGO bricks cake. Up to this point, my only experiences with cake-making have been: Prepare boxed cake, frost and “decorate”. So I searched for some instructions and found two articles in particular that seemed the most simple and accessible. The first one has a photo of a professional cake posted along with the recipe, complete with crisp fondant seams and “unrealistic” stacking. The second has the same instructions but with a photo that more accurately matches the methods so I could get a better idea of what to expect.

Here is my end result:

LEGO Bricks Cake!

Please excuse the smeary frosting mess! When you’re done at 2AM, you’re done (and very, very tired). Still, I was pleasantly surprised with how smoothly things went, considering my lack of experience going into it. My son was really excited when he saw the cake the next day, and his little friends at the party thought it was pretty neat, too!

I decided to make this tutorial to expand on the simple instructions and include some tips and tricks that I figured out along the way. There’s no step-by-step photos, but that just gives me an excuse to make diagrams! Hopefully everything is explained and detailed in a way that’s understandable. So when you think, “There’s no way I could make a LEGO bricks cake,” I say to you, “You most certainly CAN make a LEGO bricks cake!”

Tip #1 – Start Early.
This whole fiasco lasted about 6 hours, though at least half of that time was spent waiting for the cake in the freezer, and not including the initial hour-or-so of preparing and baking the cake mix. This is not really a “day-of” cake, unless you wake up at 6AM and the party’s not until 4:00 in the afternoon. My advice would be to start sometime after lunch the day before the party, and refrigerate the finished cake overnight.


  • 1 box cake mix, any flavor (Betty Crocker SuperMoist is recommended)
  • Water, oil and eggs as required for cake mix
  • 2, 1-lb. containers vanilla (white) frosting
  • Paste food colors, for whichever colors you want the blocks to be.
  • 10 marshmallows (regular sized, not mini or jumbo)

You will also need a cookie sheet, cooling rack, serrated knife, four separate smaller platforms (cardboard covered with wax paper will work fine), bowls and knives for frosting, and a larger platform for the finished cake (large enough to fit a 13×9 or 1/4 sheet cake).


  • Prepare cake mix according to box directions for a 13×9 cake pan.
  • Bake, and allow to cool at room temperature for 10-20 minutes (until pan is cool enough to handle).
  • When cooled, turn the cake out onto a cookie sheet (flat side of the cake is UP), then turn it over again onto a cooling rack (flat side of the cake is DOWN).
  • Put cake in the freezer for about 30 minutes.
  • Using a serrated kitchen or bread knife:
    1. Trim the rounded edges of cake so they are more squared.
    2. Cut cake into thirds, as equally as possible, so you have three rectangles.
    3. Trim the rounded top off the pieces, as little as possible, just to get them flat-ish and level-ish.
    4. Cut one of the rectangle pieces in half, so you have two squares.

trim/cut diagram

  • Put cake pieces back in the freezer for another 30 minutes.

Tip #2 – Keep the blocks separate.
I did not do this initially — I had all the pieces on the same tray, in and out of the freezer — and by the time I had frosted a couple of the pieces, the other ones would have thawed and started coming apart. Use a different platform for each individual cake piece. This way you can take out one piece to work on at a time, and they all stay freezing until ready to be frosted individually.

  • Crumb-coat with white frosting; this will catch and trap the crumbs to keep them out of the colored frosting layer applied later:
    1. Microwave one of the frosting tubs for 30 seconds and stir, repeat 10 second intervals (only if needed) until the frosting is pourable but not too runny. Warning: Don’t overheat or the frosting will become more of a glossy glaze.
    2. Take the cakes out of the freezer one-at-a-time and spread a thin layer of frosting evenly on all sides.
    3. Try to smooth it out as much as possible, and square off the corners as much as you can.
    4. After frosting, return cakes to freezer.

crumb coat

  • Leave crumb-coated cakes in the freezer for another 30 minutes.
  • While cakes are in the freezer, prepare your frosting colors:
    1. Combine remaining frosting from first tub with the second tub and microwave again, about 20-30 seconds, just to soften it all.
    2. Separate all into 3 equal parts and put into bowls, then take one of the bowls of thirds and split the frosting in half and into an additional bowl.
    3. Mix food color paste into frosting based on what colors you want your bricks. The two smaller portions of frosting will be for the square sections, the larger portions for the rectangles.
  • You want them to be bright and bold! Suggested colors are RED, YELLOW and BLUE, and you could leave a bowl of WHITE, or you could make some bright GREEN or ORANGE.
  • Stay away from BLACK! Solid black frosting stains horribly, and turns your lips, mouth, tongue and teeth black!

Tip #3 – You can’t get good primary colors with liquid food color!
You would need to use a whole bottle of red food color drops to get the true-red frosting, and it would pretty much be ruined with bitter flavor. If you want to get the bright, bold primary colors for your LEGO bricks, you’re going to need to get paste food colors or something along the lines of more professional-grade food colors or packaged pre-colored frosting.

  • You will want to heat your colored frosting as needed so that it has a nice smooth spreading consistency. Microwave for 10-20 seconds and stir well.
  • Remove one rectangle cake piece from the freezer, frost with one of the colored large portions of frosting (try to smooth it out as much as possible, and square off the corners as much as you can), and return to freezer.
  • Repeat with the next rectangle and other large portion of frosting.
  • Repeat with one of the square cake pieces and small portion of frosting.
  • Repeat with the other square piece and small portion of frosting.

Save the leftover frosting! You will need it for the marshmallows.

  • Leave frosted cakes in the freezer for another 30 minutes to 1 hour, until the frosting is frozen enough so you can pick up the pieces to move them.
  • When the frosting is frozen enough:
    1. Take out the first rectangle cake piece and carefully remove it from the temporary platform. You may need to “trim” around the bottom of the cakes so that the frosting doesn’t stick and pull off the cake.
    2. Use wax paper or a large spatula to lift the cake and place it in position on your large “finished” platform. See diagram below for placement.
  • Repeat steps 1 & 2 for the remaining pieces, stacking the last square. (see diagram)

stacking diagram

No need to put the cake in the freezer from this point on. But the frosting will begin to thaw and get tacky so avoid touching it when placing your marshmallow pegs.

  • Cut the marshmallows in half crosswise.

I have no idea how to cut marshmallows without smashing and deforming them. I tried straight edge and serrated knives, frozen and thawed marshmallows, but ended up using scissors. Just do your best to reshape them after cutting.

  • Frost the marshmallows with corresponding colors and place on the bricks. (refer to diagram above if necessary)

Tip #4 – Lose the knife and use your fingers!
Your hands are already washed, right? They’re going to get frosting on them no matter what. So you might as well save yourself some hassle and just apply the frosting on the marshmallow halves with your fingers. Its easier and the results are smoother and more even.

Now You’re Done!

  • Cover loosely (a tinfoil tent works well) and refrigerate overnight if necessary.

I was worried that all the freezing would make the cake taste funny and that refrigerating overnight would make the marshmallows stale, but everything was still delicious the next day! The cake was moist and fluffy (I used Betty Crocker SuperMoist cake mix) and the marshmallows were soft and spongy.

Now, go an make your LEGO bricks cake!
Let me know how it turns out!

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One thought on “Tutorial: Basic LEGO Bricks Cake

  1. Amy says:

    Helpful! Thanks for the great info as I attempt to make a Lego cake for my son’s birthday.

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