Preheat oven to 350°F (177℃). Line a square 9x9" baking dish with a parchment paper sling or spray with baking spray.
In a large microwave-safe bowl, combine the bittersweet chocolate and butter. Microwave on high in 20-30 second bursts, stirring well between bursts of heat until melted and smooth. Whisk in cocoa powder.
4 ounces bittersweet chocolate, ¾ cup salted butter, ½ cup (42g) unsweetened cocoa powder
Add granulated sugar and brown sugar, whisking well to combine. Add eggs and vanilla, beating well either by hand with a whisk or using an electric mixer.
1 cup (200g) granulated sugar, ½ cup (100g) light brown sugar, 3 large eggs, 2 teaspoons vanilla extract
Sprinkle the flour and salt over the brownie batter and gently stir it in with a wooden spoon or spatula.
¾ cup (106g) all-purpose flour, ½ teaspoon salt
Pour the brownie batter into the prepared baking dish and spread into the corners. The batter will be thick.
Bake for 30 to 35 minutes, until a toothpick inserted into the center of the brownies comes out mostly clean with only a few crumbs attached. Be sure not to overbake. Cool before topping.
To make the filling, melt 4 tablespoons of butter in a large skillet over medium-low to medium heat. Add the chopped kataifi and toast, stirring frequently until evenly toasted and golden brown, about 10-15 minutes. Reduce the heat if the kataifi starts to darken too quickly and be careful not to burn it.
4 Tablespoons salted butter, 7 to 8 ounces kataifi
Add 12 ounces of the pistachio cream (reserving the remaining 1 ounce for drizzling over the finished brownies) and tahini to the warm kataifi. Stir well to combine and evenly coat the toasted kataifi. Gently spread the warm filling over the cooled brownies in an even layer.
13 ounces pistachio cream, 3 Tablespoons tahini
Heat the heavy cream in a microwave-safe bowl for 60 seconds until bubbling. Pour over the chopped chocolate in another bowl and let it sit for 3-4 minutes without stirring. Once the chocolate has had a chance to melt, stir until completely smooth, then pour the ganache over the pistachio kataifi filling and spread into a thin, even layer.
2 cups chopped milk chocolate, ½ cup heavy cream
Warm the remaining 1 ounce of pistachio cream in a microwave-safe bowl for 20 seconds, then drizzle over the ganache layer. Chill in the fridge for 30 minutes to set the ganache before cutting the brownies into squares.
Store in an airtight container for up to 5 days on the counter or in the fridge.