Preheat the oven to 350 degrees F and lightly grease a 9X13 baking pan.
To Make the Cookie Bars: In a large mixing bowl, beat softened unsalted butter and granulated sugar until light and fluffy. Add large egg and vanilla extract. Mix for 1–2 minutes until well combined.
Stir in all-purpose flour, baking powder, and salt. Mix just until combined, being careful not to overmix.
Then spread the dough into the 9X13 pan and press the dough down slightly. Bake for 10-13 minutes. Be careful not to over bake. The middle should be just set and the edges should have a very slight color on them. Then move the pan to a wire rack to allow the bars to cool completely.
To Make the Frosting: In a large mixing bowl, beat softened unsalted butter and powdered sugar until smooth. Add milk and vanilla extract. Beat for 1–2 minutes until the frosting is light and fluffy. If desired, tint the frosting with food coloring until the desired color is reached.
Spread the frosting on top of the cooled cookie bar and top with the sprinkles (optional).
Then the bars are ready to slice, serve and enjoy!