In a medium sized microwave safe bowl, melt together the butter and 90g dark chocolate chips in 30 second intervals, stirring in between each interval until the mixture is smooth.
Immediately whisk in the cocoa powder and sugar.
Add in the egg and vigorously whisk the mixture for 2 minutes (this will help with dissolving the sugar and with getting a shiny top!)
Add in the flour and salt. use a rubber spatula to lightly fold the flour into the ingredients until just a few flour streaks remain. Then, gently fold in the remaining chocolate chips.
Reserve ~2 tbsp. of brownie batter for swirling on top. Pour the remaining brownie batter into your prepared pan.
Pour the cheesecake into an even layer on top of the brownie batter. Dollop the reserved brownie batter on top of the cheesecake layer, then use a knife to gently swirl it together.
Bake the brownies for 28-33 minutes, until they are set on the edges and only slightly wobble in the center.
Allow the brownies to fully come to room temperature, then slice, serve, and enjoy!