Preheat the oven to 180C / 160C Fan / Gas Mark 4. Grease a 20 x 24cm baking tin and line with parchment paper.
Set a heatproof bowl over a saucepan of simmering water, making sure the bottom of the bowl doesn't touch the water. Add the butter and the dark chocolate and allow to melt. Remove from the heat and stir until smooth. Set aside and leave to cool slightly.
Beat the eggs, sugar and vanilla in a mixing bowl with an electric whisk on maximum speed for 3-5 minutes until the mixture is pale and doubled in volume. Gently fold in the smooth chocolate mixture until incorporated.
Sieve the flour, salt and cocoa powder into the chocolate mixture, then gently fold in.
For the cheesecake layer, beat together all the ingredients together until smooth.
Pour 3/4 of the brownie mix into the prepared tin. Spoon the cheesecake mix randomly over the brownie layer and dollop on the remaining brownie mix. Glide a knife through the layers, creating a swirl pattern.
Sprinkle on the Dr. Oetker Extra Dark Chocolate Chips.
Bake for 30-35 minutes until the middle is set and the edges are just beginning to pull away from the sides of the tin. Remove from the oven and leave to cool before slicing.