Adding Butter to Whisked Cakes

Butter may or may not be called for in génoise and biscuit cakes; if not added carefully, it quickly knocks the air from the beaten eggs. Some cooks like the butter to be pourable but still creamy, others melt it completely or flavor it by cooking it until it is nut brown. In any case, the butter must be cool before it is folded into the batter with the last portion of flour. Sometimes a small portion of batter is added to the butter, mixed and folded back into the remaining batter.