No, I am not talking about a box cake. I have a recipe for a yellow cake that I love very much, but I also need to expand my cake options to include sponge cakes. Is there a way to turn my regular yellow cake into a sponge cake? Like leave out the butter and fold in the egg whites? 

why would you though — just get a great chiffon cake recipe — I mean you control the flavor anyway — use the same flavorings — don’t you think?

i’ve been baking/decorating for — as a home baker over 60 years and as a pro 40+ — it’s not in my genes to try to manipulate a recipe into something else it wasn’t designed to be —

it can be difficult enough to get a good recipe when you’re playing with a full deck — if you try to start out with the cards already in play how do you know what to do next ?? 

to answer your question, no I have no idea — 

best to you on your baking journey

Not sure what kind of sponge cake you are talking about but most sponge cakes use whole eggs and butter. The method of mixing and ratios may vary.  There are various kinds if sponge cakes. It sounds like you are describing an angel food cake when you talk about leaving out butter and egg yolks. I am confused.