How do I add butter to a sponge cake?
In examining sponge cake recipes, I've noticed that some call for adding the melted butter with the flour.  Some call for adding it afterwards.  

The important thing is to fold in that butter in a way that preserves the network of bubbles that was created while whipping your eggs (unsure if your recipe called for whipping whole eggs with sugar or yolks with sugar and egg whites separately, but both will be creating a bubble structure integral to letting your cake rise).  Ratio mentions that folding in flour helps to preserve that network (as long as you don't overhandle).

If you over handle while folding - and you may have tipped it over the line when adding butter - you will destroy that bubble network and your cake won't rise as you'd like.  It's also possible to under-handle, which will result in only a thin layer of sponge on top with a thick buttery cake underneath.