Prep: 10 minutes, Bake: 30 minutes, Chill: 4 hours.
1. Preheat oven to 350°. Grease and flour a 13- x 9-inch baking dish.
2. Beat butter and sugar at medium speed with an electric mixer until fluffy; mix in eggs and 1/2 teaspoon vanilla. Combine flour and baking powder; add gradually to butter mixture, stirring to blend. Pour batter into prepared dish, and bake at 350° for 30 minutes or until wooden pick inserted in center comes out clean. Pierce cake with a fork all over.
3. Combine the 3 milks, and pour on top of cake; cool to room temperature. Cover and refrigerate until well chilled, at least 4 hours or overnight.
4. Beat whipping cream, powdered sugar, and remaining vanilla at medium-high speed with an electric mixer until thick; spread over cake, and serve.