Preheat oven to 350°. Combine brown sugar, granulated sugar, tahini, and butter in the bowl of a stand mixer fitted with paddle attachment; beat at medium speed 2 minutes. Add egg and vanilla; beat 1 minute. Add flour, baking soda, and salt; beat at low speed just until combined. Mix in chocolate. Form dough into 24 equal balls; flatten slightly. Arrange on 2 parchment paper-lined baking sheets. Bake at 350° for 12 minutes, rotating pans halfway through.