I’ve been trying to baking genoise cake but the batter always deflates when I add the melted butter. So I first heat up eggs and sugar on a bain marie (water bath) on the stove to about 40C and then I whip it up until I could make ribbons that slowly fall back into the batter. I shift the cake flour and fold it into the mixture and then mix some of the batter to my melted butter and milk mixture before pouring it back into the batter mixture. That’s when it always deflates. Could anyone explain why this happens? Thank you!