For the Cookie Bars: Preheat oven to 375 degrees F. Grease a 9×13 inch baking pan with cooking spray. Set aside. (I like to line my pan with parchment paper then spray lightly with grease to easily remove the bars after they've baked and cooled.)
Combine flour, baking powder and salt in a medium bowl. Set aside.
In a large bowl or stand mixer, cream together the butter and sugar until light and fluffy, about 2 minutes. Add the egg and combine well. Add the sour cream and vanilla and beat until combined. Slowly add the flour mixture until combined.
Press mixture into the bottom of the greased pan with your hands. You can spray a little cooking spray on your hands so batter doesn’t stick to them.
Bake for 17-20 minutes only! Or just until edges become slightly golden. Remove from oven and let cool completely before frosting.
Frosting: In a medium bowl, beat together the butter, 2 cups of powdered sugar and milk until combined. Slowly add in the remaining two cups of powdered sugar until combined. Then add the vanilla and salt. Drop several drops of desired color of food dye into the frosting and beat until creamy and desired color.
Frost bars with frosting and sprinkle with lots of sprinkles! Slice and serve