I made a Lego cake for Maggie's 6th birthday. I used the "recipe" and instructions from BHG and I'm happy to say it turned out great. Pound cake, marshmallows and frosting made from 2 lbs powdered sugar and 3/4 c. milk. Plus, lots of Wilton food color gel.
The frosting felt a little like finger paint as I was dripping it on, but it turned out really nice looking. It hardened up, making the cake pieces easy to move onto the platter.