Add melted butter to the basic whisked sponge mixture to enrich the batter and make it more moist. Take care that the butter is thoroughly cooled after melting and add it to the whisked mixture slowly, after the flour has been folded in. Bake the cake as soon as possible after mixing or batter may deflate.
Melt 20 g unsalted butter and let cool. Pour slowly over the surface of the whisked mixture.
Gently fold in the butter, cutting through the mixture with a spatula so as not to knock air out of it, until evenly incorporated.