Preheat oven to 350f degrees. Line a 9x9" baking pan with parchment paper.
Beat together butter and sugar until fluffy, then beat in egg and vanilla. Finally, beat in flour and baking powder, a little at a time, until you have a thick batter. Gently stir in the sprinkles.
Spread batter into it prepared baking pan.
Bake at 350f degrees for 20 minutes or until golden brown. Once it's finished baking, take cookie bar out of the oven and allow to cool fully. Once it is cooled, carefully lift the sides of the parchment paper to pull the whole cookie bar out of the baking pan.
To make the frosting, blend together powdered sugar, butter, cream cheese, vanilla and milk with an electric mixer. Spread frosting generously on cookie bar, and top with additional sprinkles.
Cut into squares and serve immediately or let frosting set for a firmer feel.