Banana Boston Cream Pie

Preheat oven to 350 °F. Lightly butter 2 (8-inch) round cake pans and dust with flour, knocking out excess. Sift together flour, baking soda, baking powder, and salt. Beat together butter and sugar in mixer at high speed until lightened, about 5 minutes. Add yolks 1 at a time, beating well after each addition, then beat in vanilla. Add half of flour mixture and mix at low speed until just blended. Add buttermilk, mixing until just combined, then add remaining flour mixture, mixing at low speed until smooth. Transfer batter to a large shallow bowl. Clean mixing bowl and beaters. Beat whites with a pinch of salt in mixer until they just hold stiff peaks, then fold into batter gently but thoroughly. Divide batter between pans, smoothing tops. Bake in middle of oven until cake is springy to the touch and a tester comes out clean, 15 to 20 minutes. (Layers will be about 3/4 to 1 inch high.) Cool in pans on racks 5 minutes. Run a sharp thin knife around edges, then invert onto racks and cool completely.