Our boy wanted a Lego Cake for his birthday. Only problem, parent's can't bake.
Solved by buying five ready made basic, un-iced, long-life sponge cakes from the local supermarket that were roughly cubed in shape (approx 4x5x15 cm). The basic idea below could be adapted to whatever shape cake you can find to buy, or indeed you could make suitable cakes yourself. We needed to feed 12 small boys and this number of cakes of that size was about right.
Then we set to work with the design. To make it realistic we got out the real Lego and made a number 6 out of Lego on a Lego base, with different coloured blocks. For a different age you could use the same approach and make your own digit 3, 4, 5, 7 etc with real Lego as a template.
Then we measured up the dimensions of the Lego blocks and scaled these up to the size of our purchased cakes. So a Lego block that had a ratio of width to length of 1 to 1.5 became a cake section with dimensions width 5cm and length 7.5cm. The height of our purchased blocks was slightly too much compared to real Lego, but we decided not to cut the cake down any further, and no-one noticed. We then simply cut the cakes to size.
For the icing, we bought ready made fondant icing. The packets for this say what surface area they will cover, so it is easy to work out how much to buy once you have your basic design and have done your basic sums. We could only buy small 250g packets (all white), and needed six of these to cover our cake Lego blocks.
To make it more interesting we coloured the icing with food colouring. You simply knead in the colouring until it is evenly distributed.
Prepare the cakes for the icing by using a pastry brush to brush on a small amount of warmed jam. This acts as glue. Roll out the icing to a few mm thick, then drape over the cake block. Use a knife to trim to size. Then roll a sausage with the spare icing and cut small sections to make the dots to go on top of the Lego. Use a tiny amount of jam to stick the dots to the cakes.
Once your individual Lego cakes are prepared, simply follow your design to lift the blocks into place. We used a real (large) Lego base covered in foil to add some extra authenticity, but a normal cake board, or chopping board covered in foil would be fine.