Preheat the oven to 200C/180C Fan/Gas 6. Line an 8" Square Pan.
Start by melting the dark chocolate: break up the dark chocolate and melt in the microwave in 20 second intervals, stirring in between to make sure it doesn’t burn.
Combine the melted butter, granulated sugar and light brown sugar in a large bowl/bowl of a stand mixer – on a medium speed for 3-5 minutes.
Add in the eggs and vanilla and mix again. Then, checking first that the chocolate isn't so hot as to cook the eggs, add it in and keep mixing.
Add the flour and cocoa powder and fold in gently. Pour the batter into the prepared tin.
Make the cheesecake layer by combining all ingredients in a small bowl. Pour this mixture over the brownie layer.
Bake for 35-40 minutes, until the top is no longer 'wobbling'. Scatter over nuts if using. Let cool on a wire rack before slicing and serving.
