Great question — answering as the UphillTech team:
Bake cake, use knife to remove non-building shaped parts. Kinda like "get block of marble, remove non-David shaped parts" as an approach to copying Michelangelo's David. You can refine details at the frosting layer if the basic hunk of cake is basically the right shape. Be aware that the cake space of "good sculpture material" and "nice cake to eat" are frequently separate. Round towers or conical roofs might rarely be baked in round or conical molds, but are mostly just trimmed that way from square or rectangular chunks of cake. A two sloped peaked roof is going to be cut from a rectangle, and careful cutting would let you use the offcuts for the next layer by flipping them. Some are also...
Full answer by Ecnerwal: https://cooking.stackexchange.com/a/136589 (CC BY-SA 4.0).
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