Get Ready: Preheat your oven to 350°F (177°C). Line an 8x8-inch baking pan with parchment paper, leaving an overhang to easily lift the brownies later.
Make the Easy Brownie Base: In a heatproof bowl, gently melt the chopped dark chocolate and butter together. You can use a double boiler or microwave in short bursts, stirring until super smooth and shiny. In a separate bowl, whisk the sugar and eggs with a mixer until they're light and fluffy. Carefully fold the melted chocolate mix into the egg mixture – be gentle to keep that fluffiness! Sift in the flour, cocoa powder, and baking powder, then fold until just combined. Stir in the extra chopped chocolate or chocolate chips. Pour this delicious batter into your prepared pan and bake for about 35 minutes, or until a toothpick comes out mostly clean. Let it cool completely – patience is key for the best brownies!
Prepare the Crunchy Pistachio Filling: Gently pull apart the shredded phyllo dough (kadayif) into small strands with your hands. In a large skillet, melt the butter over medium heat. Add the kadayif and toast, stirring constantly, until it turns golden brown and smells wonderful. Take it off the heat. Stir in the pistachio cream and tahini until everything is nicely coated and sticky. Let it cool down a bit.
Whip Up the Silky Ganache: Heat the heavy cream in a small saucepan until it just starts to bubble around the edges – don't let it boil! Pour the hot cream over your chopped chocolate in a bowl. Cover and let it sit for about 15 minutes. Then, stir gently until it's smooth, glossy, and perfectly mixed. Let it cool slightly to thicken up a bit.
Assemble Your Masterpiece: Once your brownie base is completely cool, spread the pistachio kadayif filling evenly over the top. Next, pour the chocolate ganache over the filling and spread it out into a smooth layer. Garnish with chopped pistachios and a sprinkle of flaky sea salt if you like. Pop it in the fridge for 4-6 hours, or until it's fully set and firm – this makes slicing much easier!