1. Center a rack in the oven and preheat oven to 350°F. Butter two 8 or 9-inch round cake pans and line the bottom of each pan with a round of buttered parchment or wax paper. Put the pans on a baking sheet.
To Make Cake:
1. Sift together flour, baking powder and salt. Whisk together milk and egg whites in a medium bowl. Put sugar and lemon zest in a mixer bowl or another large bowl and rub them together with your fingers until the sugar is moist and fragrant. Add the butter and, working with your fingers until the sugar is moist and fragrant. Add butter and, working with the paddle or whisk attachment, or with a hand mixer, beat at medium speed for a full 3 minutes, until the butter and sugar are very light. Beat in the extract, then add one third of the flour mixture, still beating on medium speed. Beat in half of the milk-egg mixture, then beat in half of the remaining dry ingredients until incorporated. Add the rest of the milk and eggs, beating until the batter is homogenous, then add the last of the dry ingredients. Finally, give the batter a good 2 minute beating to ensure that it is thoroughly mixed and well aerated. Divide the batter between the two pans and smooth the tops with a rubber spatula.
