Preheat oven to 350°F and line an 8x8" pan with parchment strips and nonstick spray.
Melt the butter in a saucepan over medium heat. Remove from heat and immediately whisk in the chocolate chips and cocoa powder until smooth.
In a large bowl using a rubber spatula or stand mixer with paddle attachment, stir the eggs, sugar, vanilla and salt. Pour in the tahini and fold in to combine.
Now fold in the flour, stir until just incorporated. Remove 1/3 of the tahini mixture to a small bowl, set aside. Add the melted chocolate mixture and other 1/2 cup chocolate chips to the remaining tahini mixture, folding and stirring until batter is well combined.
Spread the chocolate batter into the prepared pan and smooth to the edges.
Drop dollops of the reserved 1/3 tahini batter all over the top of the chocolate batter and use a knife or chopstick to swirl it in slightly.
Bake for 30 minutes, or until toothpick inserted comes out clean or with a few moist crumbs. Let tahini brownies cool in the pan for at least 1 hour before cutting.
To prevent brownies from drying out too quickly, store in covered container in refrigerator