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.