Vanilla to Chocolate?
 

I found a wonderful vanilla cupcake recipe. I am thinking it would be
nice to work it into a chocolate recipe. Before I start experimenting
with no plan in mind, I thought I'd ask. Is there a formula for
changing to chocolate? What I'm thinking is perhaps there is some rule
that says....add "x" amount of cocoa powder and reduce the flour by "y".

Any ideas of how to start?

thanks
marcella