Start by whisking together the flour, baking powder, and salt in a medium bowl. In a separate large mixing bowl, cream the softened butter and sugar until light and fluffy. Add the egg and vanilla extract, blending well. Gradually incorporate the dry ingredients into the wet mixture, mixing until a smooth dough forms. Add a few drops of green food coloring and knead it gently into the dough.

Divide the dough into two equal portions, shape each into a disc, and wrap them tightly in plastic wrap. Refrigerate for at least 1 hour.

Preheat the oven to 350°F (175°C) and line a baking sheet with parchment paper. Roll out one disc of dough to about ¼-inch thickness and use a shamrock-shaped cookie cutter to create cookies. Transfer them to the prepared baking sheet, leaving space between each cookie. Sprinkle with sanding sugar, if desired.

Bake the cookies for 8-10 minutes, until the edges are lightly golden. Let them cool on the baking sheet for 5 minutes before moving them to a wire rack.