Espresso Chocolate Chip Shortbread Cookies

1 tbsp instant espresso powder
1 tbsp boiling water
8 oz. unsalted butter, room temperature
2/3 cup confectioners’ sugar
1/2 tsp vanilla extract
2 cups all-purpose flour
4 oz. bittersweet chocolate, finely chopped OR 3/4 cup store-bought mini chocolate chips
confectioners’ sugar, for dusting (optional)

Dissolve espresso powder in boiling water. Set aside to cool to tepid. Beat butter and confectioners’ sugar together on medium speed for about 3 minutes, until the mixture is very smooth. Beat in vanilla and espresso, then reduce mixer speed to low and add flour, mixing only until it disappears into the dough. Don’t work the dough much once the flour is incorporated. Fold in the chopped chocolate or chips with a sturdy rubber spatula. Using the spatula, transfer dough to a gallon-size zip-loc bag. Put bag on a flat surface, leaving the top open, and roll the dough into a 9 x 10 1/2 inch rectangle that’s 1/4 inch thick. As you roll, turn the bag occasionally and lift the plastic from the dough so it doesn’t cause creases. When you get the right size and thickness, seal the bag, pressing out as much air as possible, and refrigerate the dough for at least 2 hours, or for up to 2 days. Preheat the oven to 325°F. Put the plastic bag on a cutting board and slit it open. Turn the firm dough out onto the board (discard the bag) and, using a ruler as a guide and a sharp knife, cut the dough into 1 1/2-inch squares. Transfer the squares to the baking sheets and carefully prick each one twice with a fork, gently pushing the tines through the cookies until they hit the sheet (okay, I didn’t poke the cookies and they’re fine). Bake for 18 to 20 minutes, rotating the sheets from top to bottom and front to back at the midway point (I didn’t do that either – I bake one sheet at a time). The shortbreads will be very pale–they shouldn’t take on much color. Transfer the cookies to a rack. If you’d like, dust the cookies with confectioners’ sugar while they are still hot. Cool the cookies to room temperature before serving. Makes about 3 dozen.