Can I substitute vanilla extract for cocoa powder in a cupcake recipe?
The recipe you've found looks like "crazy cake" or "wacky cake" (or "depression cake"), which is a fairly common egg-free cake. I've made it a number of times for a friend with an egg allergy and it's a very good vegan cake.

To make a vanilla version, leave out the cocoa powder and increase the vanilla extract slightly (1.5 teaspoons). There are a wide array of variations on this blog -- chocolate is the "original" (and most common version that I have seen), but vanilla, spice, lemon, and more all seem to be possibilities.