- Working with a stand mixer, preferably fitted with a paddle attachment, or with a hand mixer in a large bowl, beat the butter and sugar together on medium speed for about 3 minutes, until the mixture is very smooth. Beat in the vanilla (and espresso, if you are using), then reduce the mixer speed to low and add the 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 with a rubber or silicone spatula.
- Using the spatula, transfer the soft, sticky dough to a gallon-size zipper-lock plastic bag. Put the bag on a flat surface, leaving the top open, and roll the dough into a 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.
- Position the racks to divide the oven into thirds and preheat the oven to 325 F/160C. Line two baking sheets with parchment or silicone mats.
- 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 – 1.5 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.
- Bake for 15-17 minutes, rotating the sheets from top to bottom and front to back at the midway point. 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.
P.S. To double the fun, try this delicious chocolate chip cake!