Preparation

• For rectangular cookies, lightly butter the bottom of a straight-sided 9×13-inch baking pan. For wedges, lightly butter two 9-1/2-inch fluted tart pans with removable bottoms.
• Combine the flour, cornstarch, and salt in a medium bowl. Dissolve the instant espresso into the vanilla.
• In a stand mixer fitted with the paddle attachment, beat the butter, sugar, and vanilla-espresso mixture on medium speed until smooth and creamy, 2 to 3 minutes. Add the flour mixture and the chopped chocolate and mix on low speed until the dough begins to form moist clumps, about 1 minute.
• Turn the dough out into the prepared pan (if making wedges, divide the dough between the two pans). Using lightly floured fingertips, press the dough into the pan(s) to form an even layer.
• 对于矩形饼干，请使用长凳或削皮刀将面团一直切成40杆。用叉子的尖齿，每个条刺刺两次一直穿过两次，将孔均匀地和对角线隔开。根据需要将尖齿轻轻面粉，以防止面团粘住。
• For wedges, cut each pan of dough all the way through to form 12 wedges. With the tines of a fork, prick the wedges three times all the way through, spacing the holes evenly over the dough.
• 烤箱加热时冷冻面团。将架子放在烤箱的中心，然后将烤箱加热至325°F。
• Bake the dough, rotating the pan(s) halfway through, until the top(s) looks dry and golden-brown, 30 to 40 minutes. Transfer the pan(s) to a rack. Using a paring knife or bench knife, immediately re-cut the cookies along the lines. Cool completely before removing them from the pan(s).

Make Ahead Tips

Once the dough is pressed into the pan and cut, it can be wrapped and refrigerated for 1 day or frozen for up to 1 month. If freezing the dough, thaw it overnight in the refrigerator before baking.

Baked and cooled cookies can be layered between sheets of parchment or waxed paper in an airtight container and stored at room temperature for up to 4 days.

For chocolate-dipped shortbread, melt 6 oz. chopped bittersweet or white chocolate with 1 tsp. canola or vegetable oil. Brush any lose crumbs from cooled shortbread and dip two-thirds of each cookie into the chocolate. Dry on a cooling rack until chocolate firms, about 45 minutes.