1/2 cup old-fashioned rolled oats
1 1/2 cups unbleached all-purpose flour
1/4 cup cornstarch
2/3 cup confectioners’ sugar
1/2 teaspoon table salt
14 tbsp unsalted butter, cold, cut into 1/8-inch-thick slices
Makes 16 wedges
Use the collar of a spring form pan to form the shortbread into an even round. Mold the shortbread with the collar in the closed position, then open the collar, but leave it in place. This allows the shortbread to expand slightly but keeps it from spreading too far. Wrapped well and stored at room temperature, shortbread will keep for up to 7 days.
Adjust oven rack to middle position and heat oven to 450 degrees. Pulse oats in spice grinder or blender until reduced to fine powder, about ten 5-second pulses (you should have ¼ to 1/3 cup oat flour). In bowl of stand mixer fitted with paddle attachment, mix oat flour, all-purpose flour, cornstarch, sugar, and salt on low-speed until combined, about 5 seconds. Add butter to dry ingredients and continue to mix on low-speed until dough just forms and pulls away from sides of bowl, 5 to 10 minutes.
Place upside-down (grooved edge should be at top) collar of 9- or 9 1/2-inch spring form pan on parchment-lined rimmed baking sheet (do not use spring form pan bottom). Press dough into collar in even 1/2-inch-thick layer, smoothing top of dough with back of spoon. Place 2-inch biscuit cutter in center of dough and cut out center. Place extracted round along side spring form collar on baking sheet and replace cutter in center of dough. Open spring form collar, but leave it in place.
Bake shortbread 5 minutes, then reduce oven temperature to 250 degrees. Continue to bake until edges turn pale golden, 10 to 15 minutes longer. Remove baking sheet from oven; turn off oven. Remove spring form pan collar; use chef’s knife to score surface of shortbread into 16 even wedges, cutting halfway through shortbread. Using wooden skewer, poke 8 to 10 holes in each wedge. Return shortbread to oven and prop door open with handle of wooden spoon, leaving 1-inch gap at top. Allow shortbread to dry in turned-off oven until pale golden in center (shortbread should be firm but giving to touch), about 1 hour.
Transfer baking sheet to wire rack; cool shortbread to room temperature, at least 2 hours. Cut shortbread at scored marks to separate and serve.