Preheat the oven to 180C/350F/Gas 4. Line an 8x8-inch square baking pan with foil or parchment paper, and set aside.

Place the butter, sugar, cocoa powder, and salt into a microwave-safe bowl, and heat for 1 minute. Stir well, then microwave for another minute. The mixture should very warm to the touch.

Add the eggs and vanilla, and mix well until combined. Fold in the flour until fully combined.

Reserve 1/4 cup of the batter, and set aside. Pour the remaining batter into the prepared pan, and spread out evenly. Bake for 15 - 20 minutes, or until the centre of the brownies is mostly set.

Allow to cool in the pan on a wire rack whilst preparing the cheesecake.

Using a handheld or stand mixer fitted with the paddle attachment, beat the cream cheese and sugar until combined. Beat in the egg, then mix in the vanilla extract, and flour.

Pour the mixture over the brownie layer and spread out evenly. Spoon the reserved brownie batter on top, and using a toothpick, create a swirl effect.

Bake for 20 - 30 minutes or until the cheesecake centre is almost set, but still a little jiggly. Allow to cool to room temperature, then refrigerate for 4 - 5 hours. Cut into bars, and serve.