Preheat your oven to 180°C (350°F). Grease a 9-inch (23 cm) square baking pan and line it with parchment paper, allowing the paper to overhang the sides to create a sling for easy removal. If you’d like, secure the edges of the parchment to the pan with binder clips to hold it in place.

In a heatproof bowl set over a saucepan of gently simmering water (ensuring the bottom of the bowl doesn’t touch the water), combine the chopped dark chocolate and butter. Stir frequently until both are melted and the mixture is completely smooth. Alternatively, you can melt the chocolate and butter together in the microwave in 30-second bursts, stirring thoroughly between each interval until smooth and fully combined.

Once melted, sift the cocoa powder over the chocolate-butter mixture and stir until well blended.

In a separate large mixing bowl, whisk together the eggs, caster sugar, and brown sugar using a hand mixer on medium-high speed for about 2–3 minutes, or until the mixture becomes paler and increases in volume. This step can also be done in a stand mixer fitted with the whisk attachment.

Add the melted chocolate mixture to the whipped eggs and sugars and mix to combine. Stir in the salt and vanilla until fully incorporated.

Finally, add the sifted flour and fold it in by hand until the batter is smooth and free of any lumps.

Pour the finished brownie batter into the prepared pan and tap it gently on the counter several times to remove any air bubbles. Let the batter settle for a minute or two, then tap again to ensure no bubbles remain.

Place all the cheesecake layer ingredients into a medium bowl. Using a hand mixer on medium speed or a whisk, mix everything together until the cheesecake mixture is smooth, creamy, and lump-free.

Spoon approximately three-quarters of the cheesecake mixture over the brownie batter in dollops, then use a toothpick or the tip of a spoon to swirl it through the brownie layer. For a nice marbled effect, gently scoop down to bring up a little of the brownie batter and swirl it into the cheesecake.

Add the remaining cheesecake mixture on top and swirl again to your desired pattern.

Bake the brownies in the preheated oven for 22 to 28 minutes, beginning to check at the 20-minute mark. You’ll know they’re ready when the top and edges look set and a skewer inserted into the center comes out with a few moist crumbs. Aim slightly underbaked rather than overbaked to preserve a fudgy texture.

Once baked, remove the pan from the oven and set it on a wire rack. Let the brownies cool completely in the pan, then lift them out using the parchment sling. Slice into squares using a sharp knife for clean edges.

Store any leftover brownies in an airtight container at room temperature or in the fridge for up to four days.