Add the granulated sugar and, using an electric mixer on high speed, beat for 5-7 minutes. You want a thick, pale mixture that has nearly tripled in volume and falls in ribbons when lifted from the whisk. This technique incorporates air into the eggs, a key step for a fluffy, sponge-like texture.
Test the ribbon stage: Lift the beaters; if the batter falls back into thick ribbons that slowly dissolve into the mixture, you’re ready for the next step. In a small saucepan, gently heat the milk and unsalted butter over medium-low heat. Stir until the butter is fully melted, then remove from the heat and let cool slightly. The milk should be warm but not scalding, as high temperatures can scramble the eggs when added.
Warming the milk and butter helps create a finer crumb in the finished cake. Sift the flour mixture one last time directly over the whipped eggs and sugar. Do this in three batches, folding gently with a rubber spatula after each addition. Use a light hand and slow movements to incorporate the flour without deflating the aerated egg mixture.
