Brownie Layer:

1. Preheat your oven to 350°F (180°C). Lightly grease a 9-inch (23 cm) square baking pan and line it with parchment paper, leaving extra paper hanging over the edges to create a sling for easy removal. You can secure the parchment with binder clips if you like.
2. Place the dark chocolate and butter in a heatproof bowl set over a pot of gently simmering water, ensuring the bowl doesn’t touch the water. Stir frequently until the mixture is fully melted and smooth.
   Optional: You can also melt them in the microwave using 30-second bursts, stirring well after each interval until smooth.
3. Sift the cocoa powder over the melted chocolate mixture and stir until fully blended.
4. In a separate large bowl, whisk together the eggs, granulated sugar, and brown sugar using a handheld electric mixer on medium-high speed. Beat for 2–3 minutes, until the mixture becomes paler and slightly thickened. (A stand mixer fitted with the whisk attachment works too.)
5. Pour in the melted chocolate mixture and mix to combine. Add the salt and vanilla, mixing until everything is incorporated.
6. Add the flour and fold it in gently by hand until the batter is smooth and free of lumps.
7. Spread the batter into the prepared pan. Tap the pan firmly on the counter a few times to release any air bubbles. Let it rest for a minute or two, then tap again to ensure the batter is settled and bubble-free.

Cheesecake Layer:

1. Place all the ingredients for the cheesecake layer in a medium bowl. Combine either with a hand mixer on medium speed, or with a whisk, until the mixture is smooth and lump free. 
2. Dollop about ¾ of the cheesecake mixture on top of the brownie batter, and swirl in using a toothpick or a spoon – I like to scoop down a tiny bit and bring up bits of brownie batter for a nice swirl. 
3. Add the rest of the cheesecake mixture and swirl as desired.
4. Bake the cheesecake brownies for 22 to 28 minutes, checking after 20 minutes. You are looking for the edges and top of the brownie to be set, and when a skewer is inserted into the middle, it should come out with a few moist crumbs attached. You are best to lean toward under baking than over baking. 
5. Remove the brownies from the oven and place the pan on a wire rack. Leave to cool in the pan then remove using the parchment paper sling. Slice into squares using a sharp knife. 
6. Store leftovers in an airtight container at room temperature or in the fridge for up to four days.