Heat to 350 degrees F (175 degrees C). Grease and flour pan; set aside. In a small bowl, stir together flour, baking powder and salt; set aside. In large bowl, beat sugar and butter on medium-high speed until light and fluffy, 2 to 3 minutes. Reduce speed and add the eggs and egg whites one at a time, beating well after each addition. Stir in vanilla. Add the flour mixture and milk beating until just combined; scraping bowl often. Beat on medium speed 1 minute; stir in rainbow sprinkles. Bake 30-35 minutes, until toothpick inserted comes out clean. Cool 15 minutes in pan. Invert onto cooling rack and cool completely before frosting.
Buttercream Frosting: Beat butter, powdered sugar, sugar and vanilla until smooth. If desired, stir in 1-2 drops of food coloring.
**For chocolate frosting, add 1/4 cup cocoa powder and 1 tsp. milk to recipe, omitting food coloring.