In a large bowl, mix the butter with sugar until light and fluffy; beat in egg and vanilla. Whisk together flour, matcha green tea powder, baking powder and salt. Gradually stir into the butter mixture until dough comes together.

Divide the dough in half, form into discs and wrap each in plastic wrap. Refrigerate for 1 hour. Let stand at room temperature until soft enough to roll out, about 15 minutes.

Preheat oven to 350 °F. Line baking sheet with parchment paper, set aside.

Place the dough on a lightly floured surface and roll to 1/4 inch thick. Use shamrock shaped cookie cutter to make cookie shapes, re-rolling any scraps. Repeat process with the remaining dough.

Place cookies onto the prepared baking sheet and bake for 10 to 12 minutes or until edges are light brown. Let cookies cool for a minute on baking sheets then transfer to racks and let them cool completely.

Recipe yields 1 1/2 - 2 dozen cookies, depending on the size of your cookie cutter.