01 - Preheat the oven to 350°F. Line a 9x13-inch baking pan with parchment paper, leaving an overhang on the sides for easy removal.
02 - In a large bowl, beat the 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. Beat until fully incorporated and smooth.
04 - In a separate medium bowl, whisk together the all-purpose flour, baking powder, and salt until evenly distributed.
05 - Gradually add the dry ingredient mixture to the wet ingredients, mixing on low speed just until combined. Avoid overmixing to prevent tough texture.
06 - Transfer the dough to the prepared pan. Press it evenly across the bottom using clean hands or the back of a spatula.
07 - Bake for 18-20 minutes, or until the edges are lightly golden and the center appears set. Do not overbake. Cool completely in the pan before frosting.
08 - While the bars cool, beat the softened butter in a clean bowl until creamy and smooth.
09 - Gradually add the powdered sugar, alternating with 2 tablespoons of milk. Beat on medium-high speed until the frosting becomes smooth and fluffy. Add additional milk if needed to achieve spreadable consistency.
10 - Mix in the vanilla extract and pinch of salt until incorporated. Add food coloring at this stage if desired and mix until uniform color is achieved.
11 - Spread the frosting evenly over the completely cooled cookie bars using an offset spatula. Add sprinkles immediately while the frosting is still soft.
12 - Lift the entire sheet from the pan using the parchment paper overhang. Transfer to a cutting board and slice into 24 equal-sized bars.