01 - Preheat oven to 350°F. Line a 9x13-inch baking pan with parchment paper, allowing overhang on edges for easy removal later.
02 - In a large bowl, beat softened butter and granulated sugar together until light and fluffy, approximately 2-3 minutes using an electric mixer.
03 - Beat in the egg and vanilla extract until fully incorporated into the mixture.
04 - In a separate bowl, whisk together flour, baking powder, and salt until evenly distributed.
05 - Gradually add dry ingredients to wet mixture, mixing just until combined. Be careful not to overmix as this can create tough bars.
06 - Evenly press the dough into the prepared baking pan, creating a uniform layer.
07 - Bake for 18-20 minutes until edges are lightly golden and center appears set. Avoid overbaking to maintain soft texture. Cool completely in pan before frosting.
08 - Beat softened butter until creamy. Gradually incorporate powdered sugar, alternating with milk, beating until smooth and fluffy consistency is achieved.
09 - Mix in vanilla extract and pinch of salt. Add food coloring now if desired and mix until uniform color.
10 - Spread frosting evenly over completely cooled cookie bars. Add sprinkles immediately while frosting is still sticky if using.
11 - Use parchment paper overhang to lift bars from pan. Cut into 24 equal squares and serve.