01 - Whisk together flour, baking powder, and salt in a medium bowl. Set aside for later use.
02 - Beat softened butter and granulated sugar in a large bowl until light and fluffy, approximately 2 minutes.
03 - Add egg and vanilla extract to the butter mixture, beating until fully incorporated.
04 - Gradually add dry ingredients to the wet mixture, mixing just until combined. Avoid overmixing.
05 - Divide dough into two discs, wrap tightly in plastic wrap, and refrigerate for at least 1 hour.
06 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
07 - On a lightly floured surface, roll one dough disc to 1/4-inch thickness. Cut out shamrock shapes using cookie cutter.
08 - Place cookies 1 inch apart on prepared baking sheets. Bake for 8-10 minutes until edges are barely golden.
09 - Let cookies rest on baking sheets for 5 minutes, then transfer to wire rack to cool completely.
10 - Whisk powdered sugar, 2 tablespoons milk, corn syrup, and vanilla until smooth. Adjust with additional milk for thick but pourable consistency.
11 - Stir in green food coloring until desired shade is achieved. Decorate cooled cookies with icing and allow to set before serving.