Irish Shortbread Cookies are classic shortbread cookies cut into shamrock shapes and glazed with green icing. Use a Ziploc bag to roll out dough for easy clean-up.
Combine 1 cup unsalted butter, room temperature and 3/4 cup powdered sugar into a large bowl. Beat on medium-high speed until fluffy. Add 1 tablespoon vanilla extract and beat until combined.
Combine 2 cups all-purpose flour and 1 teaspoon baking powder in a separate bowl. Add slowly to the large bowl with butter using low speed. Stop beating as soon as the flour mixture is incorporated. Overbeating will result in toughness.
Form the dough into a slightly-flattened ball. Place into a zippered plastic bag. Chill about 30 minutes. With bag cracked open slightly to allow air to escape, roll dough until evenly distributed throughout bag. Place back into refrigerator until firm--at least another 30 minutes or up to 2 days. Can be frozen at this point.
Preheat oven to 350˚F (177˚C). Line baking sheets with parchment paper or silicone mats.
Remove cookie dough from fridge and carefully cut two sides of bag with kitchen shears or scissors, leaving one side intact. Lay top layer of plastic back and use cookie cutter to make cookies. Because the dough is cold and firm, they should peel off the plastic easily. Place onto prepared cookie sheet. Re-roll scraps after replacing plastic over top of new ball and refrigerate a few minutes until firm again. Repeat as needed until all dough is used.
Bake 10 to 12 minutes, until light brown around the edges. Time will vary greatly depending on the size and shape of cookies. Check early and often. Cool 5 minutes before removing to cooling rack and wait until cold before icing.
Icing
Whisk together the 2 cups powdered sugar, sifted, if lumpy, 3 tablespoons milk, and 1/2 teaspoon vanilla or almond extract. If too thick, add more milk a few drops at a time. If too thin, add more powdered sugar or let sit for a while. Tint with 2 + drops green food coloring.
Ice cookies and allow to rest until dry. I prefer to turn cookies upside down and dip them into the icing just enough to cover the surface. Goes faster, although a little messy, until you get the hang of it. Store cookies in an airtight container at room temperature for up to 1 week or tightly wrapped in aluminum foil or plastic wrap for 1 month in the freezer.