Position a rack in the centre of the oven and heat to 325°F.
In a small bowl, stir the tahini and confectioners’ sugar until combined. Set aside.
In a large heatproof bowl, melt butter and chocolate in the microwave for about 2 minutes, stir using a whisk halfway through. Whisk until smooth.
Whisk in cocoa, set aside.
In a separate and smaller bowl, whisk together eggs, sugar, vanilla and salt. Whisk into the cocoa mixture in the larger bowl.
Using a spatula or wooden spoon, stir in the flour until just combined.
Transfer to the cast iron skillet, and smooth the top.
Dollop the tahini mixture on top of the brownie dough, and swirl together with the tip of a knife. Bake until the tahini no longer looks wet and the top cracks a bit, 35 to 38 minutes. Let cool slightly before serving. Sprinkle the salt once cooled for a few minutes.
Serve with vanilla ice cream, of course.
