Preheat oven to 325°F (165°C). Line an 8x8-inch square pan with parchment paper, leaving overhang on two sides, and lightly grease.
In a medium saucepan over low heat, melt chopped dark chocolate and butter together, stirring constantly for 3-4 minutes. Remove from heat and cool for 5 minutes.
Add both sugars to the cooled chocolate mixture and whisk vigorously for 1 minute until glossy and thickened.
Add eggs one at a time, whisking each in completely before adding the next. Stir in vanilla extract and whisk for 30 seconds until smooth.
Whisk together flour, cocoa powder, and salt. Gently fold into chocolate mixture just until no dry pockets remain, being careful not to overmix.
Pour batter into prepared pan, spread evenly, and tap pan on counter 2-3 times. Bake 28-32 minutes until edges are set but center looks slightly underdone. Cool in pan 30 minutes, then refrigerate 30 minutes.
Cut kataifi into 1-2 inch pieces. Melt 4 tablespoons butter in a large skillet over medium-low heat, add kataifi, and toast for 15-18 minutes, stirring frequently until deep golden brown.
Remove skillet from heat and stir pistachio cream, tahini, and a pinch of salt into the warm toasted kataifi until evenly coated. Let cool 10 minutes.
Spread the pistachio kataifi mixture evenly over the cooled brownies, pressing gently to adhere. Refrigerate 10-15 minutes.
Place chopped milk chocolate in a bowl. Heat heavy cream until just simmering, pour over chocolate, and let sit 2 minutes. Whisk until smooth and glossy.
Pour ganache over pistachio layer and spread evenly. Optional: drizzle warmed pistachio cream on top and swirl with a toothpick. Sprinkle with chopped pistachios and refrigerate 1 hour until set.
Lift brownies from pan using parchment overhang. Cut into 16 squares with a sharp knife, wiping clean between cuts. Let sit at room temperature 10-15 minutes before serving.