Start by making the brownies:

Preheat oven to 325F/160C. Grease an 8-inch round or square pan or line with parchment paper.

In a large heat-proof bowl, put chocolate and butter. Melt them in the microwave in 20 second-intervals, stirring between each interval, until melted (or set over a saucepan of simmering water, stirring occasionally). Add sugar and whisk until combined. Add eggs, one at a time, and whisk just until combined. Add vanilla and stir. Do not over mix. Sift the flour and salt over the chocolate mixture and fold using a rubber spatula, just until mixed.

Set aside ½ cup of the batter. Pour the rest of the batter into the prepared pan and smooth the top. Bake for 5-8 minutes, just until set. Set on the counter to cool while you make the cheesecake.

To make the cheesecake:

Melt chocolate in the microwave as explained above. Set aside to cool.

Using a mixer, beat cream cheese on medium speed until smooth. Add granulated sugar and beat just until light and smooth. Scrape the sides and bottom of the bowl as needed. Add cocoa powder and beat until combined. On low speed, beat in the egg just until combined. Slowly add melted chocolate and beat until combined. Pour into the pan, on top of the brownie batter.

Take the reserved ½ cup brownie batter and spoon dollops on top of the cheesecake batter. With a knife or skewer, swirl the two batters together to get a marble effect.

Bake for 35-45 minutes, or until a toothpick inserted into the center comes out with a few moist brownie crumbs, and the cheesecake filling is just beginning to brown (it’s ok if the cheesecake layer isn’t fully set – it will once cooled). Let the brownies cool completely and then refrigerate them for at least one hour until they are firm enough to cut.

Store brownies in an airtight container in the fridge for up to 5 days. Brownies can also be frozen for up to 2 months. Thaw frozen brownies in the fridge for a few hours before serving.