How do I add butter to a sponge cake? I made a Genoise following James Peterson's recipe in Baking, but without the optional butter.  I used an electric hand mixer. The cake came out fine, as in the picture.  The second time I decided to add the butter and the whole thing deflated.  What did I do wrong?  How do I add the butter to the batter?

Peterson's recipe calls for beating eggs and sugar for 20 minutes with a handheld to ribbon stage, folding in the flour with a spatula, and tempering the melted butter with 1/5 of the batter before mixing it to the rest.
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.