Bake this a day before serving so it soaks up the creamy syrup. Make mini cakes with a round cutter. Top with whipped cream and a grating of nutmeg.
Preheat oven to 350°. Beat butter at medium speed with an electric mixer until creamy; add granulated sugar, beating until light and fluffy. Add egg yolks, 1 at a time, beating until blended after each addition. Stir together flour, baking powder, and salt. Add flour mixture to butter mixture alternately with milk, beginning and ending with flour mixture. Beat at low speed until blended after each addition. Scrape bowl as needed. Stir in vanilla. Wash and dry beaters.
Beat egg whites at high speed until stiff peaks form. Stir about one-third of egg whites into batter; fold in remaining egg whites in 2 batches. Spoon batter into a lightly greased (with cooking spray) 13- x 9-inch pan.
Bake at 350° for 23 minutes or until a wooden pick inserted in center comes out clean. Cool in pan on a wire rack 10 minutes.
Pierce top of cake several times with a long wooden pick. Whisk together condensed milk, evaporated milk, and liqueur. Gradually pour mixture over warm cake, about 1/2 cup at a time. Allow mixture to soak into cake before adding more. Let cake stand at room temperature 3 hours. Cover and chill 8 to 12 hours.
Beat heavy cream and powdered sugar at medium-high speed with an electric mixer until stiff peaks form; spread on cake.