Adjust oven rack to middle position and heat oven to 450 degrees. Pulse oats in a spice grinder or blender until reduced to a fine powder, about 10 pulses (you should have 1/4 to 1/3 cup oat flour). Using stand mixer fitted with paddle, mix oat flour, all-purpose flour, pistachios, cornstarch, sugar, and salt on low speed until combined, about 5 seconds. Add butter to dry ingredients and continue to mix until dough just forms and pulls away from sides of bowl, 5-10 minutes.

Place upside-down (grooved edge should be at top) collar of a 9- or 91/2-inch springform pan on parchment paper-lined baking sheet (do not use springform 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 alongside springform collar on baking sheet and replace cutter in center of dough. Open springform collar, but leave it in place.

Bake shortbread for 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 oven off. Remove springform pan collar; use chef’s knife to score surface of shortbread into 16 even wedges, cutting halfway through shortbread. Using wooden skewer, poke about 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 sheet to wire rack; let shortbread cool to room temperature. Cut shortbread at scored marks to separate.

Melt 8 ounces finely chopped bittersweet chocolate in microwave at 50 percent power for 2 minutes. Stir chocolate and continue heating until melted, stirring once every additional minute. Stir in additional 2 ounces finely chopped bittersweet chocolate until smooth. Carefully dip base of each wedge in chocolate, allowing chocolate to come halfway up cookie. Scrape off excess with your finger and place on parchment paper-lined rimmed baking sheet. Refrigerate until chocolate sets, about 15 minutes.