With the oven rack in the middle position, preheat oven to 350 degrees F. Line an 8 x 8 inch square pan with enough parchment paper to create an overhang of at least two inches on two sides.

Melt the butter and semisweet chocolate together in a bowl in the microwave. Microwave in 15 second intervals, stirring in between until melted. Set aside to cool.

In a small bowl, stir together 1/4 cup of the tahini and the powdered sugar. Set aside.

Sift together the flour, cocoa, and salt. Set aside.

In a large bowl, briskly whisk together the eggs and sugars for about a minute or until mixture becomes pale and creamy. Whisk in the vanilla, followed by the remaining 2 tablespoons of tahini and the reserved butter/melted chocolate mixture. Use a wooden spoon or a spatula to gently stir in the dry ingredients. Mix until just combined then pour into the prepared pan.

Use a teaspoon to drizzle the prepared tahini-powdered sugar mixture overtop. To do so, scoop some of the mixture onto your spoon and drizzle it in swirls over the top of the mixture. Next, use a toothpick or the tip of a knife to gently make swirls in the batter. Do just a few swirls in each section as you make your way across the mixture. Be careful not to over-swirl or you won't get the cool marbled look. 

Bake 17-20 minutes or until a toothpick comes out with just a few crumbs when inserted in the middle of the brownies. Do not wait until it comes out with no crumbs. 

Cool in the pan for at least 2 hours at room temperature or 1 hour in the fridge.