Preheat the oven to 450°F. Line an 8-inch round non-stick springform pan or high-sided 8-inch round cake pan with 2 overlapping 16x12-inch sheets of parchment, making sure parchment comes at least 2" above top of pan on all sides.

Make the brownie batter. Combine dark chocolate and butter in a heatproof bowl and heat in the microwave in 30 second bursts until completely melted and smooth. Add the egg, sugar and salt and whisk until the batter is glossy and smooth. Sprinkle over the flour and fold it in gently. Spread the brownie batter evenly into the base of the lined pan and place it in the fridge while you make the cheesecake batter.

Make the cheesecake batter. Add cream cheese to a large bowl and beat with an electric hand mixer on medium-high speed until very smooth. Gradually add in sugar and continue mixing until very well combined and sugar is dissolved and there are no lumps. Scrape down the bowl at least 2 times during this process.

Beat in eggs one at a time, mixing well before adding the next. Scrape down sides of bowl between each addition. Beat in vanilla and salt. Add the melted chocolate mixture and sifted cocoa powder and mix it in.

Warm the cream in the microwave for 20-30 seconds just so it is lukewarm, but not hot. Very gradually beat in the slightly warm cream until combined and the batter is smooth and silky.

Pour batter through a sieve (to remove any lumps) and into prepared pan and bake for 22-25 minutes until deeply dark brown on top. Do NOT over-bake. The cheesecake should still be very jiggly when you take it out the oven.

Transfer pan to a cooling rack and let cool completely in the pan. Cover and refrigerate for at least 8 hours or overnight.

Carefully peel away parchment from sides of cheesecake. Slice into wedges and serve at room temperature.