Preheat oven to 350 degrees. Line an 8-inch square pan with foil and lightly coat with butter. Set aside.
In a medium saucepan, melt butter over medium heat. Remove from heat and add chocolate and cocoa powder, whisking until smooth. Set aside.
In a large bowl, whisk the eggs, sugar, vanilla and salt until slightly thickened. Whisk in the tahini. Fold in the flour and mix until just incorporated.
Transfer 1/2 cup of batter to a small bowl, and set aside. Add the chocolate mixture to the remaining tahini mixture and fold until fully combined.
Pour the batter into the prepared pan, spreading evenly. Dollop the reserved tahini mixture over the top, then swirl batters together.
Bake until the edges are set but center is moist--about 28-32 minutes. Cool in the pan for 30 minutes before moving to a wire rack to cool completely. (The longer they cooler, the better they cut). Cut into 2 inch squares