Preheat oven to 375 degrees F.
Lightly grease a 9X13-inch baking pan with cooking spray or line with parchment paper and grease. Set aside.
Sugar Cookie Bars:
In a large bowl, cream together the butter, granulated sugar and powdered sugar until light and fluffy. Add the eggs and vanilla and mix for 1-2 minutes. Stir in flour, baking powder and salt. Mix until combined. Optional: If you want sprinkles in cookie bars, add in ½ cup and mix in.
Press the sugar cookie dough into an even layer in the bottom of the prepared pan.
Bake for 15-18 minutes. TIP: The bars should have hardly any color around the edges and barely be set in the middle, Don't overbake! Let the bars cool completely before frosting.
Cream together the butter and powdered sugar until combined, add a little milk if needed if not mixing well, then add in the vanilla, milk or cream, and food coloring (if using), and mix until the frosting is light and fluffy, 1-2 minutes. Add in ½ cup sprinkles and mix into frosting, add more sprinkles if you want.
Spread the frosting over the cooled bars and top with more sprinkles.