01 - Preheat oven to 350°F. Line a 9x13-inch baking pan with parchment paper, leaving an overhang for easy removal.
02 - In a medium bowl, whisk together flour, baking powder, and salt. Set aside.
03 - In a large bowl, beat butter and sugar with an electric mixer until light and fluffy, about 2–3 minutes.
04 - Add eggs one at a time, beating until combined. Mix in vanilla extract.
05 - Gradually add flour mixture, alternating with milk, beating on low speed until just combined.
06 - Spread dough evenly into the prepared pan.
07 - Bake for 18–20 minutes, or until edges are lightly golden and a toothpick inserted in the center comes out clean. Do not overbake.
08 - Cool completely in the pan on a wire rack.
09 - Beat butter until creamy. Gradually add powdered sugar, milk, and vanilla, beating until smooth and fluffy. Add green food coloring, a few drops at a time, until desired shade is reached.
10 - Spread frosting evenly over cooled cookie base. Decorate with shamrock sprinkles or green sanding sugar if desired.
11 - Lift out of the pan using parchment overhang and cut into bars.