I have a few recipes from my mother-in-law, who I always thought baked from scratch... Come to find out that EVERY SINGLE ONE of her ecipes start with "1 yellow cake mix" or "1 white cake mix"

The problem with this is that I don't recall ever buying a cake mix. I am 25 and grew up baking with my grandmother, who never used a cake mix. I have every other ingredient you could want to make 30 or more cakes, but no CAKE MIX!!!

Can anyone please tell me how to substitute this ingredient?!?