I just tried baking biscuit de savoie with melted butter - 4 tbsp for half recipe of Rose’s biscuit de savoie recipe (1 9 inch pan) . It was delicious, moist, and absolutely spongy and tender. It is almost identical to Julia Child’s biscuit au buerre or butter spongecake recipe from Mastering the Art of French Cooking Vol 1. While I have baked genoise successfully i had not been pleased with the syruping part - the cake always seemed dry or i tasted too much syrup. I will be using this butter enhanced recipe as a base for all my cake layers going forward.