Preheat the oven to 350℉. Grease an 8x8 baking pan with butter and line it with a parchment paper sling.
Add butter and sugar to the bowl of a stand mixer fitted with a paddle attachment (or use an electric hand mixer and large mixing bowl). With the mixer on high speed, mix the ingredients together for 1-2 minutes, until light and fluffy.
Add the egg and vanilla extract and mix on medium speed until egg is fully incorporated.With the mixer speed on medium-low, carefully add the flour, baking powder, and salt. Mix to combine, stopping when there are no visible streaks of flour in the cookie dough.
Add dough to the prepared baking dish, then press down on it to flatten. Transfer to the center rack of preheated oven and bake for 20 to 22 minutes or until edges are light golden brown. **The top may look slightly glossy; this is okay. The cookies will continue cooking as they cool down.
Remove from oven and transfer baking dish to a wire rack to cool for at least 30 minutes before frosting.
Use an offset spatula or butter knife to spread the frosting evenly over the bars. Decorate with sprinkles, then slice and serve.
Notes
Line the baking dish with a sling to make it easier to remove the cookie bars. Cut 2 strips of parchment paper, long enough to extend over the the sides of the pan. Lay them down so they cross one another in the middle.