Preheat oven to 350 degrees F. Butter and line the bottom of a 9-inch springform pan with parchment. Wrap the springform pan with foil twice. Place the springform pan in a roasting pan.
Melt the chocolate in a heat proof bowl over a pan of simmering water until smooth, stirring occasionally. Alternatively, you can melt in the microwave in 30 second intervals, stirring in between until melted. Set aside.
In a stand mixer fitted with the paddle attachment, beat cream cheese on medium speed until smooth, about 2 to 3 minutes. Add in the sugar, cocoa powder, and beat until smooth, scraping the sides of the bowl once. At low speed beat in the eggs, 1 at a time. Slowly pour in the melted chocolate and mix until fully combined. Beat in the sour cream just until incorporated.
Pour the filling into the prepared pan and smooth the top.
Bring a kettle of water to a boil for the water bath.
Put the pan and roasting pan into the oven, then slowly pour in the boiled water into the roasting pan, so that it goes about 1-inch up the sides of the springform pan.
Bake the cheesecake until the centre is set and the top looks slightly dry, about 35 to 40 minutes.
Let the cheesecake cool in the pan for about 5 minutes, then using a paring knife, cut around the edges to loosen the sides. Allow the cheesecake to cool completely in the fridge.