How to Make It
Preheat oven to 350°F. Whisk together flour, cocoa, and salt in a medium bowl; set aside.
Beat together sugar, butter, oil, and vanilla in a large bowl with an electric mixer on medium speed until well blended, about 3 minutes. Add egg, and beat until combined. Add flour mixture; beat on low speed until just combined.
Shape dough into 36 balls (1 tablespoon each). Arrange dough balls 2 inches apart on 2 baking sheets lined with parchment paper. Lightly press each dough ball to flatten slightly, and make a “thumbprint” impression in dough balls by gently pressing your thumb or the handle of a wooden spoon in center of each ball. Bake at 350°F until set, about 10 minutes. Remove from oven, and cool on baking sheets set on a wire rack for 3 minutes. Transfer cookies directly to wire rack to cool completely, about 30 minutes.
Meanwhile, place chocolate chips in a medium microwavable bowl. Microwave at high until chocolate melts, about 1 minute, stirring after 30 seconds. Whisk tahini into melted chocolate until smooth. Spoon about 1 teaspoon chocolate mixture into impression of each cookie. Sprinkle with crushed peppermints. Let stand until set, about 1 hour (or chill 20 minutes to quickly set filling). Store in an airtight container up to 3 days.