In a medium bowl whisk together the flour, baking powder, and salt. Set aside.
In a large bowl, beat 1 cup of the butter, the granulated sugar, and the brown sugar until creamy and smooth.
Beat in the egg and vanilla.
Add the flour mixture and mix until combined completely.
Mix in the sprinkles.
Press the dough evenly into a 9 X 13 pan. Bake for 15-18 minutes or until the center is slightly firm and the edges are a little golden brown.
Let the pan of cookie cool completely.
While it cools, make the buttercream. Beat the ½ cup of butter with the powdered sugar. Add the 1 teaspoon of vanilla. Splash the milk in 1 tablespoon at a time to reach the desired consistency. Add the food coloring and beat the frosting until smooth.
Spread the frosting over the cooled cookie bars and then top with more sprinkles. You can very gently press the sprinkles into the buttercream so they'll set.
Recipe by Sugar Dish Me at https://www.sugardishme.com/frosted-funfetti-cookie-bars/