Step 1

Heat the oven to 350ºF with a rack in the middle position. Line an 8-inch-square baking pan with 2 pieces of foil with excess hanging over the edges on all sides. Lightly coat with butter.

Step 2

In a medium saucepan over medium heat, melt the butter. Remove from the heat and add the chocolate and cocoa, whisking until smooth.

Step 3

In a large bowl, whisk the eggs, sugar, vanilla and salt until slightly thickened, about 1 minute. Whisk in the tahini. Fold in the flour until just incorporated. Set aside ½ cup of the mixture.Add the chocolate mixture to the remaining tahini mixture and fold until fully combined.

Step 4

Pour the batter into the prepared pan, spreading evenly. Dollop the reserved tahini mixture over the top, then swirl the batters together with the tip of a paring knife.Bake until the edges are set but the center remains moist, 28 to 32 minutes. Cool in the pan on a wire rack for 30 minutes. Use the foil to lift the brownies from the pan.Cool on the rack for at least another 30 minutes before removing from foil. The longer the brownies cool, the more easily they cut. Cut into 2-inch squares.