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 mixing bowl, beat softened butter and granulated sugar with an electric mixer until pale and fluffy, approximately 2–3 minutes.
03 - Mix in the egg and vanilla extract until thoroughly incorporated, scraping down the bowl sides as needed.
04 - In a separate medium bowl, whisk together flour, baking powder, and salt until evenly distributed.
05 - Gradually incorporate dry ingredients into wet mixture, mixing on low speed just until flour disappears. Avoid overmixing to prevent tough texture.
06 - Transfer dough to prepared pan and press evenly across the surface using clean hands or an offset spatula.
07 - Bake for 18–20 minutes until edges turn light golden brown and center appears set. Remove promptly—overbaking yields dry bars. Cool completely in the pan before frosting.
08 - Beat softened butter in a clean bowl until creamy and smooth. Gradually add powdered sugar while mixing, alternating with milk to achieve spreadable consistency.
09 - Blend in vanilla extract and salt. If using food coloring, add drops until desired shade is reached. Whip until light and fluffy.
10 - Spread frosting evenly over completely cooled cookie bars. Sprinkle with decorations immediately, before frosting crusts.
11 - Using parchment overhang, lift the entire slab from the pan. Cut into 24 uniform squares with a sharp knife.