1. In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.

2. In a large bowl, beat the softened butter and sugar until light and fluffy, about 2–3 minutes.

3. Add the egg and vanilla extract to the butter mixture and beat until fully combined.

4. Gradually add the dry ingredients to the wet ingredients, mixing until a soft, slightly sticky dough forms.

5. Divide the dough in half, wrap each half in plastic wrap, and refrigerate for at least 30 minutes.

6. Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper.

7. On a lightly floured surface, roll out one half of the dough to about 1/4-inch thickness.

8. Use a shamrock-shaped cookie cutter to cut out the cookies and place them on the prepared baking sheets.

9. Bake for 8–10 minutes or until the edges are lightly golden.

10. Let the cookies cool on the baking sheets for a few minutes before transferring them to a wire rack to cool completely.

11. In a small bowl, whisk together powdered sugar, milk, and vanilla extract until smooth.

12. Add a few drops of green food coloring to achieve your desired shade. If the icing is too thick, add a little more milk.

13. Once the cookies are completely cool, spread the green icing onto each shamrock-shaped cookie.

14. Add sprinkles if desired. Allow the icing to set before serving or storing the cookies.