coarsely chopped bittersweet chocolate (at least 65 percent cacao)
kosher or sea salt
Heat oven to 375° and set racks in upper and lower thirds of oven.
In a food processor or blender, pulse 1 1/4 cups oats until very finely ground. Add cornstarch and baking powder; pulse briefly. In a large bowl, use an electric mixer to cream butter and sugars until light and fluffy. Add egg and vanilla and beat until smooth. Add flour mixture and stir until just combined. Fold in chocolate and remaining oats.
Drop dough by tablespoons, 2 inches apart, onto 2 parchment-lined baking sheets. Sprinkle with salt. Bake until edges are golden brown, about 15 minutes. Cool on sheets 5 minutes; transfer to a wire rack to cool completely.