1. Place an oven rack in the upper third of oven. Preheat oven to 350°.
2. Line an 8-inch square metal baking pan (not dark nonstick) with parchment paper; lightly coat parchment paper with cooking spray.
3. Combine sesame oil, butter, and chocolate in the top of a double boiler. Cook over simmering water until chocolate almost fully melts, stirring gently with a spatula. Remove top of double boiler; stir until chocolate fully melts. Pour chocolate mixture into a large bowl; add granulated sugar, stirring with a whisk until well combined. Add 2 eggs, stirring with a whisk. Stir in vanilla and salt. Fold in flour in two or three batches. Pour mixture into prepared pan. Combine tahini, brown sugar, and remaining 1 egg in a small bowl. Drop spoonfuls of tahini mixture onto brownie batter; swirl with the tip of a knife.
4. Bake at 350° for 30 minutes or until a wooden pick inserted in center comes out with moist crumbs clinging. Cool in pan on a wire rack for at least 15 minutes before serving.