1. Preheat oven to 350 degrees. Butter cake pans. Line bottoms with parchment paper. Butter parchment, and dust with flour, tapping out excess; set aside. Sift together flour, baking powder, and 1/2 teaspoon salt into a medium bowl; set aside.

2. Put butter and 2 cups sugar into the bowl of an electric mixer fitted with the paddle attachment; mix on medium-high speed until pale and fluffy, about 5 minutes. Mix in egg yolks, 1 at a time, and zest. Reduce speed to low. Add flour mixture in 3 batches, alternating with milk. Transfer batter to a large bowl.

3. Put egg whites into the clean bowl of the electric mixer fitted with the whisk attachment; beat on medium-high speed until foamy. Add a pinch of salt; beat until soft peaks form. Gradually add remaining 1/4 cup sugar; beat until stiff peaks form.

4. Fold one-third of the egg-white mixture into batter with a rubber spatula. Gently fold in remaining egg-white mixture until just combined; do not overmix.

5. Divide batter among prepared pans. Bake until cake is golden and a cake tester inserted into center comes out clean. Transfer to a wire rack; let cool slightly, about 20 minutes. Invert onto rack. Remove parchment; reinvert. Let cool completely. Cakes can be refrigerated, wrapped in plastic, up to 1 day.