I have posted this cake before, 
here.  This time I made it for # 3's birthday.  AND I successfully made it gluten free!
It was made to look like a lego brick, but that frosting was hard to smooth and I think the gf made it a little crumblier on the circle parts so it was hard to do.  Also my chocolate started getting cold again before I totally incorporated it so it has little 'chips' in the frosting still.  Maybe if I tempered it more, by adding the cold to the warm chocolate bowl before adding it to the rest.?
Anyway, here is the gf version *(One small change I had to make after I fixed the recipe was that I was out of eggs but had some frozen whites so instead of all the eggs I used 4 eggs and two egg whites.) 
CAKE
1 cup Butter, softened
1 cup Brown sugar, firmly packed
2/3 cup Sugar
6 Eggs
2 teaspoons Vanilla
1/2 teaspoon Salt
2 cups 
GF Flour mix
1 12 oz. package Little Bits Semi-sweet Chocolate chips, divided
FROSTING
1 cup Little bits semi-sweet chocolate chips 
3/4 cup Butter, softened
1 1/2 cups Sifted confectioners' sugar
2 teaspoons Vanilla
Cake: Preheat oven to 350 F. Grease 15 1/2 x 10 1/2 x 1" baking pan. 
Line with parchment or waxed paper. In bowl, cream butter, brown sugar, 
sugar.  Add eggs, one at a time, beating well after each addition. Add 
vanilla and salt. Mix well.  Gradually add flour. Stir in 1 cup 
chocolate bits. Spread in prepared pan. bake 350º for 20-25 minutes. 
Cool completely. 
Frosting: Melt over hot (not boiling) water, 1 cup semi-sweet chocolate 
bits, stir until smooth. Set aside. In bowl, combine butter and 
confectioners' sugar; beat until creamy. Add melted chocolate and 
vanilla; blend until smooth. 
Loosen sides of cake. Invert and peel of paper.  Trim edges if 
necessary.  Cut cake crosswise into 4 sections. Spread frosting on one 
layer. Top with second cake layer. Repeat layers. Frost entire cake with
 remaining frosting.
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Once again I did not have a 15x10 pan so I used a regular larger cookie 
sheet. this resulted in 5 layers for me.  I cut 4 across the pan then 
cut some off each end to make a 5th layer.  Probably for this reason I 
needed more frosting than the recipe made.