Preheat the oven to 350℉ and line an 8x8 baking pan with parchment paper.
In a small saucepan, melt the 6 tbsp butter and ⅓ cup chocolate chips over low heat, stirring frequently until smooth. Set aside to cool for 10 minutes, saving the saucepan for step 7).
In the bowl of a stand mixer, beat together the egg, brown sugar, ¼ cup granulated sugar, and vanilla extract until smooth and frothy (about 2-3 minutes).
While the mixer is running on low, slowly pour in the cooled chocolate butter mixture and stir until well combined. Add the flour, cocoa powder, and salt and stir until well combined.
Pour the batter into the prepared baking pan and bake for 15-17 minutes. Allow to cool completely in the pan before moving on to the pistachio layer.
Combine the pistachio cream, tahini, and ⅛ tsp salt in a medium mixing bowl, stirring until well combined. Fold in the toasted kataifi. Once the brownies are cooled, spread the pistachio kataifi evenly over the brownies, then set aside.
Make the chocolate layer by heating the ¾ cup chocolate chips and 4 tbsp butter together in the saucepan used in step 2. Heat over low until melted and smooth, then pour directly on the pistachio layer.
Drizzle 1 tbsp pistachio cream over the warm chocolate layer, then swirl with a knife or toothpick. Cover the brownies and place in the fridge for at least 1 hour until set.