In a double boiler, melt chocolate and butter together and set aside to cool. Then, preheat oven to 350 degrees and line an 8x8 pan with parchment or use a non-stick brownie pan with a removable bottom.
In a medium bowl mix flour, cocoa, cornstarch and salt. Set aside.
Using a handheld mixer on high speed, mix granulated sugar and eggs in a large bowl until light in color and silky ribbons are formed. About 1-2 minutes.
Pour cooled chocolate and butter mixture into the sugar and eggs and mix to combine.
Gently fold flour mixture into liquid mixture until just combined. Pour into prepared pan.
In a small bowl, vigorously mix tahini and confectioner’s sugar with a spoon until smooth. Dollop tahini mixture onto the brownie batter in the pan in a grid pattern of about 10 or 12 dollops. Using the spoon, swirl to connect and combine all sections while mixing lightly into the brownie batter.
Bake for 15 minutes, rotate and bang/drop the pan against the oven rack, and continue to cook another 15-20 minutes until cooked through the center.
Let the brownies cool for about 5-10 minutes, and remove from the pan. Continue to cool for about 30 minutes before cutting.