1 teaspoon granulated sugar mixed with a dash of cinnamon
Place a rack in the center of the oven. Preheat the oven to 325 degrees.
Place the flour, brown sugar, and salt in a food processor and pulse to combine. Cut the butter into tablespoons and scatter in the processor bowl. Pulse 15 to 20 times, or until the butter is incorporated and the dough starts to come together but is still crumbly.
Press the dough into the bottom of an ungreased 8- or 9-inch square metal baking pan. Prick the dough about 12 times with a fork. Place the pan in the oven.
Bake until the shortbread is golden brown, from 35 to 45 minutes. Remove the pan from the oven. If desired, sprinkle the cinnamon-sugar over the top. Cut the shortbread into squares while warm. Let cool completely before packing or serving.