Building a Lego Block Birthday Cake

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For someone like me who loves the decorating aspect of baking cakes, I surprised myself at how much of a challenge it was to come up with a fun cake idea for my son’s 9th birthday. The older he gets, the less fun the party themes become. But I couldn’t disappoint him with some store bought cake knowing how much he loves the ones I do. With my thinking cap on I decided there was no way I could do a Lego Halo Wars cake like he wanted, but I could keep things simple and turn his cake into giant Lego blocks.

In my mind, I imagined this being a super easy baking project but when it came down to icing everything I found myself very frustrated. I even got a little sloppy at one point after realizing just how long it was starting to take getting all of the pieces frosted.

To get things started I used two boxed cake mixes (yellow and chocolate) and pulled out my Ultragold bakeware to bake 2 – 13×9 cakes. Once each cake had cooled in the fridge for 30 minutes or so, each cake was then cut into thirds. The last third was cut in half to make two squares.You may notice that leveling cakes is not my strong suit (even with a cake leveling tool).

lego cake blocks

I gave each cake a quick dirty ice to keep the crumbs under control and popped them in the fridge overnight.

lego cake blocks dirty icedFor the icing I decided the basic primary colors would be bright, bold and fun. I simply took a basic buttercream recipe, tripled it and divided into 2 large bowls and 2 small bowls. Using Wilton gel colors you can create some very rich colors.

lego cake buttercream colors

The most tedious part of this project is creating the “Lego bumps”. There’s probably a more technical term for those but that’s what I can them and I’m stickin’ with it. Using jumbo marshmallows you cut each one in half, place the bottom (sticky side) on a fork and frost the edges and top. Each Lego bump sticks right onto your frosted cakes.

lego cake marshmallows

I’d like to say I love the cakes but the truth of the matter is … I got bored and slightly sloppy, missing a few edges. But that didn’t stop my son from completely loving his Lego Blocks Cakes!

lego cake blocks four colors

lego cake blocks top view

lego cake blocks candles

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  1. Sara says

    I am planning a Lego cake for my son’s bday. I am planning on mini cupcakes for the “Lego bumps”. As far as cake leveling, looks good to me…and if you need to, always add more frosting! (BTW- LOVE the term “dirty ice”. I will no longer use the term “crumb coat”. It’s “dirty ice” from now on!!!!)

      • beck says

        love cake boss and the term dirty ice was looking for ideas for a bday cake come across this blog had to comment lol good job on the cake by the way

  2. Lauralee Hensley says

    The cake frosting colors are perfect for lego blocks. What a great idea.

  3. says


    That is a great idea. I have one turning nine in February and I think this would be a great cake to make for him. He would totally love the marshmallows.

    Where does the time go? I secretly want it back, but am so glad to see them developing into REAL people.

  4. Lisa@Gardenofmany says

    Great Job Cat, I would have never thought of using marshmallows for the Lego bumps

  5. says

    This looks great, but have you ever considered throwing a cupcake party? There are entire cookbooks devoted to the art of Cupcakes. Ever had a Tiramisu cupcake? Ever gotten a group together to see who can create the funkiest cupcake? Bakery South Bay is fun and where it’s at!