Cake – Adding melted chocolate to a sponge cake batter

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.

I have never added butter to sponge sheets when making swiss rolls, jelly rolls, or anything similar in the past. It's just straight up bread flour, sugar, and eggs.

I can only think of three possible reasons why butter might be added:

1. To improve the flavour;

2. To keep the sheets moist and help prevent overbaking, as any fat would be used in cake batter; however, this is generally not a major concern in cake rolls; or

3. To help prevent it from sticking to the sheet (don't bother; if you have this problem, use a silicone mat).

Reasons #2 and #3 aren't very convincing to me, so I would only bother adding butter if you prefer the taste of swiss rolls with butter added.

I don't think butter would help with rolling at all, in fact it might make rolling more difficult on account of making the sheets more crumbly and less able to hold together under pressure.