Preheat oven to 350 F. Combine flour, baking powder, and salt in a large bowl. Beat egg yolk with 3/4 cup sugar on high speed until yolks are pale yellow. Stir in vanilla and milk. Pour egg yolk mixture over the flour mixture and stir gently until combined. Beat egg whites on high speed until soft peaks form. With the mixer on, pour in remaining 1/4 cup sugar and beat until egg whites are stiff but not dry. Fold egg white mixture into the batter very gently until just combined. Pour into greased 9×13 inch pan and spread to even out the surface. Bake at 350 F for 35-45 minutes or until toothpick stuck in middle comes out clean. Turn cake out onto rimmed platter and cool. Poke cake with fork and pour on last three milks, mixed. Let set for 30 minutes. Spread with whipped cream. Serve cold.