Preheat the oven to 350 degrees Fahrenheit (180 Celsius). Prepare an 8-inch springform pan.
Sift together the flour, brown sugar, cocoa powder, baking powder, salt, and cornstarch.
Beat in the butter until the mixture forms a sandy texture.
Add the buttermilk, eggs, and vanilla, then mix until smooth.
Pour the batter into the prepared pan and bake for thirty five to thirty eight minutes, until a tester inserted into the center comes out clean.
Let the cake cool in the pan for ten minutes, then turn it out onto plastic wrap. Wrap the warm cake and cool completely on a wire rack.
Level the top of the cooled cake if needed before assembling.
For the filling, melt the butter in a skillet over medium heat. Add the kataifi pastry and toast, stirring frequently, until golden and crisp.
Transfer the toasted kataifi to a bowl and mix in the pistachio cream, tahini if using, and a pinch of salt until fully coated.
For the ganache, combine the chocolate chips, heavy cream, and corn syrup in a microwave-safe bowl. Microwave in forty-second bursts until the cream is hot.
Let the ganache mixture stand for a couple of minutes, then stir until completely smooth.
Line the springform pan with baking paper, or use an adjustable cake ring with an acetate collar. Place the cooled cake layer inside.
Add the pistachio kataifi filling on top of the cake and level it out.
Pour the ganache over the filling, spreading it edge to edge.
Pipe the warm pistachio cream decoratively on top and sprinkle with chopped pistachios.
Chill until the chocolate sets, then release from the pan. Slice with a sharp knife, wiping the blade clean between cuts.