So I bought one for your birthday instead!
It's Kenzo's birthday soon and so we went to T&T to buy a nice fruit cake. That's not the one, but it looks similar.
How do you say "I'm not good at making cakes"?
You need to use のが which transforms a verb into a gerund. In this cake - I mean, case - we use the verb for make in short form.
to make: つくる （作る）
to make in short form: 作る (it stays the same because positive short form for verbs is the same as dictionary form)
making cakes: ケーキを作るのが
I'm not good at making cakes.
So, I buy them at T&T.