Preheat the oven to 375*F. Line a 9×13 baking dish with parchment paper or foil and spray with cooking spray, set it aside. In a large bowl whisk together the flour, baking powder, and salt, set aside.
In the body of a stand mixer with the paddle attachment cream the butter with the sugar until light and fluffy, 2 minutes. Scrape down the sides. Add the eggs one at a time, letting the first egg mix all the way in before adding the next. Add the vanilla and mix it in until combined. Add the flour mixture and stir it in until combined, scrape down the sides, and mix it in a few more times until fully mixed in. Add the sprinkles and stir them in.
Press the cookie dough into the prepared baking dish until even. Bake for 25-28 minutes until lightly golden and a toothpick inserted into the center comes out clean. Place on a wire rack to cool compltely.
While the bars cool, make the frosting by placing the butter into a large bowl with an electric hand mixture and whipping until smooth. Add in the powdered sugar a little at a time until fully mixed. Add the salt, vanilla, and milk, stir it in until combined then place the mixer on medium-high speed and whip for 3 minutes until light and fluffy.
Once the cookie is cool, take it out of the baking dish and frost the top. Add more sprinkles if desired. Cut into bars and serve.
