How to Make It
Preheat oven to 325°F. Lightly coat a 9-inch tart pan with a removable bottom with cooking spray.
Process granola and powdered sugar in a food processor until very finely ground. Add butter and pulse just until incorporated, about 7 times. Press dough into prepared pan. Bake until edges are just set, 30 to 32 minutes. While still warm, carefully remove sides from pan and cut shortbread into 8 wedges. Transfer wedges to a parchment-lined baking sheet. Return to oven and bake until cookies are firm, 8 to 10 minutes more.
Transfer cookies to a wire rack and cool completely. Dust with powdered sugar before serving.