Since no one else on instructables seems to have made any cactus cakes i can copy, this is what i've made.
For anyone else who loves cactuses and chocolate cake...
Step 1: Mixture
•bar of chocolate
•paper cake cases
This part is easy. Feel free to skip it and make your own chocolate cakes (or any other kind of cake, i just thought chocolate would look like the soil).
Pre-heat the oven to 180C.
Cream together the butter and sugar in a bowl. Whisk in the eggs and then slowly stir the flour and cocoa powder in. Leave the mixer going for a while until its completely smooth and the colour is equal.
Meanwhile, get a bar of chocolate and smash it up with the end of a rolling pin, then carefully open it and stir in the chocolate chips with a spoon. (I suppose you could just buy chocolate chips, but this is more fun)
Give the whisk to a nearby child.