1/2 cup plus 2 tablespoons old-fashioned rolled oats (not quick-cooking)
3/4 cup all-purpose flour, (spooned and leveled)
1/3 cup confectioners sugar
1/2 teaspoon salt
8 tablespoons (1 stick) cold unsalted butter, cut into pieces
Preheat oven to 325. Spread oats on a rimmed baking sheet. Toast until fragrant and lightly browned, 4 to 6 minutes. Let cool completely.
In a food processor, combine flour, sugar, salt, and 1/2 cup cooled oats; process until finely ground, 1 minute. Add butter. Pulse until mixture is the consistency of coarse meal. Transfer to an 8-inch square baking pan; press firmly into bottom. Sprinkle remaining oats on top, and press gently.
Bake until firm and lightly browned, 30 to 35 minutes. Using a paring knife, immediately score shortbread into 16 squares; then score each square diagonally into 2 triangles. On a wire rack, cool completely in pan; gently invert, and break along scored lines.