1. Preheat the oven to 375 degrees Fahrenheit. Line a 9×13 baking dish with parchment paper or foil and spray with cooking spray.
2. In a large bowl, whisk together the flour, baking powder, and salt, then set aside.
3. In the body of a stand mixer with the paddle attachment, cream the butter with the sugar until light and fluffy, about 2 minutes. Scrape down the sides.
4. Add the eggs one at a time, letting each egg mix in fully before adding the next. Add the vanilla and mix until combined.
5. Add the flour mixture and stir until combined. Scrape down the sides and mix again until fully combined.
6. Add the patriotic sprinkles and stir them in gently.
7. Press the cookie dough into the prepared baking dish until even. Bake for 25 to 28 minutes until lightly golden and a toothpick inserted into the center comes out clean. Place on a wire rack to cool completely.
8. While the bars cool, make the frosting by placing the butter in a large bowl and whipping until smooth with an electric hand mixer.
9. Gradually add the powdered sugar until fully mixed. Add the salt, vanilla, and milk, stirring until combined. Then whip on medium-high speed for 3 minutes until light and fluffy.
10. 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.