Arrange a rack in center of oven; preheat to 325°. Spray a 9" x 9" baking dish with cooking spray, then line with parchment, leaving an overhang on 2 opposite sides.
In a large bowl, prepare brownie mix according to package instructions. Transfer to prepared dish; smooth top.
Bake brownies until a tester inserted into the center comes out with a couple moist crumbs attached, 35 to 40 minutes. Let cool.
Meanwhile, in a large skillet over medium-low heat, melt butter. Add kataifi and cook, stirring often and reducing heat to medium-low if kataifi darkens too quickly, until deep golden in color, 15 to 18 minutes. Remove from heat.
Fold in tahini and 1 ¼ cups plus 2 Tbsp. pistachio cream. Let cool while brownies are baking and cooling. Spread kataifi mixture over cooled brownies.
Place chocolate in a medium heatproof bowl. In a medium pot over medium heat, heat cream until it begins to gently bubble around edges of pan. Pour over chocolate, let sit 2 minutes, then whisk until melted and smooth.
Pour ganache over kataifi mixture. Using a spoon, drizzle remaining 2 Tbsp. pistachio cream over ganache. Using a toothpick, create swirls throughout, trying not to disturb kataifi layer. Sprinkle with pistachios.
Using parchment overhang, lift out brownies and transfer to a cutting board. Cut into squares.
