How to Make It
Preheat oven to 325°F. Whisk together flour, powdered sugar, cornstarch, and salt in a medium bowl.
Beat together butter, oil, and granulated sugar in a large bowl with an electric mixer on medium speed until well blended, about 3 minutes. Beat in vanilla. Add flour mixture; beat on low just until combined.
Pat or roll dough into a 10 1/2-inch square on a baking sheet lined with parchment paper. Bake at 325°F until set and lightly browned around edges, about 30 minutes. Remove from oven, and immediately cut into 36 (1 3/4-inch) squares (a pizza cutter and ruler make this go quickly). Cool cookies completely on baking sheet, about 1 hour.
Place chocolate in a small microwavable bowl. Microwave at high until melted, about 1 minute, stirring after 30 seconds. Using a spoon, butter knife, or small offest spatula, spread chocolate over half of top side of each cookie, and sprinkle flaky salt evenly over chocolate. Let stand until chocolate sets, about 1 hour. Store in an airtight container up to 3 days.