The design is much simpler than the Wedding Cakes I have made for various people in the past. The birthday cake was only one layer, was a basic rectangle, and only had the one corner 'slice' was removable (to show the yellow cake beneath).
I designed and built this model in about a day. While it could have basically been a simple 'shell' of a cake, it is actually quite solid. The interior is about 50% full of brick, all 2x2s. This made the cake quite sturdy (and heavy -- it weighed 10 pounds), but also meant I did not have to glue much of it (only some of the extremity pieces).
The top was decorated with a few colored 1x1 plates, the 'Happy Birthday' was spelled with dark red plates, while the woman's name was spelled with medium/light blue plates.
I was also thrilled to finally have a reason to use those little plastic LEGO flame elements!
The final cake was about 12 inches by 19 inches (I had originally planned to make it smaller, but then I realized how much room the last name 'Wedderspoon' would take to spell out... not often I run across surnames as long as mine).