The powdered sugar helps create the soft, crumbly texture of these shortbread cookies. Granulated sugar gives cookies their crispness. To balance out that crispness in shortbread cookies, a bit of powdered sugar is added.u003cbr/u003eu003cbr/u003eIf you do not have powdered sugar, you can make your own by combining 1 cup granulated sugar plus 1 tablespoon cornstarch in a food processor. Pulse for 30 seconds until powdery. Store powdered sugar in an airtight container.