1/2 c. old fashioned rolled oats
1 1/2 c. (7.5 oz.) flour
1/4 c. cornstarch
1/4 c. powdered sugar
1/2 t. salt
2 T. lemon zest (about 2 grocery store-sized lemons worth)
1 T. finely chopped rosemary
14 T. butter (1.75 sticks), cold and cut into 1/8" slices
flaked salt
Put rack in middle of oven and heat oven to 450 F.
Pulse oats to a fine powder in food processor (about ten 5 second pulses).
Mix oats, flour, cornstarch, powdered sugar, salt, lemon zest, and rosemary. Add butter and beat on low until dough just forms.
Place 9" or 9.5" springform pan upside down (minus bottom) on a baking sheet. Press dough into 1/2" layer, smoothing top with the back of a spoon. Use a 2" biscuit cutter to remove a circle from the center of the dough. Place circle on the baking sheet. Open springform collar, but leave in place. Sprinkle top of dough with flaked salt.
Bake shortbread 5 minutes, then reduce oven temperature to 250 F. Continue to bake until edges turn golden, another 10-15 minutes. Remove baking sheet and turn off oven. Remove springform collar and use a chef's knife to score the top into 16 wedges, cutting halfway through the shortbread. Use a wooden skewer to poke 8-10 holes in each wedge. Return to oven and prop door open 1" with the handle of a wooden spoon. Allow to dry until pale golden in center (about 1 hour). Put baking sheet on cooling rack and let cool to room temperature. Cut on scores.
Adapted from Cook's Illustrated's Best Shortbread recipe.
