01 - Preheat oven to 350°F. Line a 9x13-inch baking pan with parchment paper, allowing overhang on sides for easy removal later.
02 - In a large mixing bowl, beat softened butter and granulated sugar together with an electric mixer until light and fluffy, approximately 2-3 minutes.
03 - Add the egg and vanilla extract to the butter mixture, beating until fully incorporated and smooth.
04 - In a separate bowl, whisk together flour, baking powder, and salt until evenly distributed.
05 - Gradually add dry ingredients to wet ingredients, mixing on low speed just until combined. Avoid overmixing to prevent tough texture.
06 - Transfer dough to prepared pan and press evenly across the bottom using your hands or a spatula, creating a uniform layer.
07 - Bake for 18-20 minutes until edges are lightly golden and center is set. Remove from oven and cool completely in pan before frosting.
08 - Beat softened butter until creamy. Gradually add powdered sugar, alternating with milk, beating until smooth and spreadable.
09 - Mix in vanilla extract and pinch of salt. Add food coloring if desired and beat until uniform color is achieved.
10 - Spread frosting evenly over completely cooled cookie bars. Top with sprinkles immediately if using, before frosting sets.
11 - Using parchment paper overhang, lift bars from pan. Cut into 24 equal squares and serve immediately or store in airtight container.