Preheat oven to 375°F.
Lightly grease a 9x13-inch baking pan.
Cream butter and sugar until light and fluffy.
Add eggs and vanilla and mix well.
Stir in flour, baking powder, and salt just until combined.
Press dough gently into an even layer in the pan.
Bake 10 to 13 minutes. Do not overbake. The bars should be barely set in the center and the edges should not be browned.
Cool completely before frosting.
For frosting, beat butter with 2 cups powdered sugar until smooth and fluffy, about 3 to 5 minutes.
Add remaining powdered sugar and vanilla.
Add milk as needed until soft and spreadable.
Spread frosting over cooled bars.
Top with sprinkles, M&M's, and white chocolate chips.
Make ahead: bake bars 1 day in advance and frost before serving.
Store covered at room temperature up to 2 days or refrigerate up to 5 days.
