01 - Preheat the oven to 350°F. Line a 9x13-inch baking pan with parchment paper, allowing excess paper to hang over the sides for easy removal later.
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 - Beat in the egg and vanilla extract until fully incorporated into the mixture.
04 - In a separate bowl, whisk together the all-purpose flour, baking powder, and salt until evenly distributed.
05 - Gradually add the dry ingredients to the wet ingredients, mixing just until combined. Be careful not to overmix, which can make the bars tough.
06 - Press the dough evenly into the prepared baking pan, using your hands or a spatula to create a uniform layer.
07 - Bake for 18–20 minutes, or until the edges are lightly golden and the center appears set. Avoid overbaking to maintain a soft texture. Cool completely in the pan before frosting.
08 - Beat the softened butter until creamy. Gradually add the powdered sugar, alternating with milk, and continue beating until smooth and fluffy.
09 - Mix in the vanilla extract and pinch of salt. Add food coloring if desired and beat until fully incorporated.
10 - Spread the frosting evenly over the completely cooled cookie bars. Add sprinkles immediately if using, before the frosting sets.
11 - Lift the bars from the pan using the parchment paper overhang. Transfer to a cutting board and slice into 24 equal squares.