Prepare the Bars:

Preheat Oven: Preheat your oven to 350°F (175°C). Grease or line a 9x13-inch baking dish with parchment paper.

Cream Butter and Sugar: In a large mixing bowl, beat the softened butter and granulated sugar together until light and fluffy.

Add Wet Ingredients: Mix in the egg, egg yolk, and vanilla extract until well combined.

Combine Dry Ingredients: In a separate bowl, whisk together the flour, baking powder, and salt. Gradually add the dry ingredients to the wet mixture, mixing until just combined.

Bake: Spread the dough evenly into the prepared baking dish. Bake for 20-25 minutes, or until the edges are lightly golden and the center is set. Allow to cool completely.

Make the Frosting:

Cream Butter: In a mixing bowl, beat the softened butter until smooth and creamy.

Add Powdered Sugar and Milk: Gradually add the powdered sugar, milk, and vanilla extract, beating until the frosting is smooth and spreadable. If using, add gel food coloring to achieve your desired shade.

Assemble the Bars:

Frost the Bars: Spread the frosting evenly over the cooled cookie base.

Decorate: Add sprinkles or decorations as desired.

Slice and Serve: Cut into squares or rectangles and serve.