It's important that everything is softened to room temperature. If you need to, place the cream cheese (with the packaging) and the egg in a bowl of hot water for 5-10 minutes and microwave the sour cream for 10-20 seconds.
Place the cream cheese with the sugar and cocoa powder in the bowl of your stand mixer, fitted with the paddle attachment (or a large bowl with a hand mixer). Turn the mixer to low speed and mix for 1-2 minutes, scraping the edge of the bowl every so often to make sure the mixture is smooth and there are no chunks of cream cheese left.
8 oz (1 brick) cream cheese, 1/2 cup (100g) granulated sugar, 2 Tbsp (10g) unsweetened cocoa powder
Add the sour cream, vanilla and salt and mix at low speed, until they're fully combined. Then add the egg and mix just until it's fully combined.
1/4 cup (60g) sour cream, 1 tsp vanilla bean paste, 1/8 tsp fine sea salt, 1 large egg
Add the melted chocolate and fold it in with a rubber spatula. Scrape the edge of the bowl well and give it one more good mix. Make sure to smooth out or remove any visible chunks.
1/3 cup (2oz) dark chocolate
Pour the cheesecake batter over the crust and place the cheesecake on a baking sheet on the middle rack of the oven. Place another medium sized pan halfway full of hot water next to it.
Bake for 30 minutes. The edges of the cheesecake should be set but the center should still be jiggly.
Turn the oven off and leave the door halfway open for 15 minutes.
Remove the cheesecake from the oven and allow it to cool to room temperature for a couple of hours.