I made a sponge cake using a receipe from an old cookbook and at one point it told me to melt and boil butter and pour the boiling butter into beaten egg-whites, let sit for a minute and then mix gently. This procedure puzzles me. Why should I pour in the butter while hot?
If you want the whole thing, here it goes (no measures, 'cause it's in Polish and uses Polish measures, where "one cup" is not what you would call "one cup"): beat egg whites with powdered sugar until stiff, pour in melted and boiling butter (or shortening), mix gently after one minute and let sit until it cools. Start mixing again and add egg yolks one by one, then lemon juice and finally flour mixed with baking powder. Mix a bit more. Bake at least 50 minutes in 160 deg C
The batter looks fine, it's baking nicely, but the long time is also strange for me, most sponge cakes that I made require just 20-25 minutes of baking. I skimmed the book, but didn't notice any other sponge cake receipe with this approach.