Preheat the oven to 325F
Slowly melt the chocolate in a double boiler (or microwave) and set aside.
Medium speed, cream the cream cheese and sugar until complete;ly smooth, scrape down the sides regularly.
Keep mixing on low speed, add the cocoa and sour cream until fully incorporated.
Keep mixing, and slowly add the chocolate, and eggs - 1 at a time.
Despite all the mixing, don't overmix!
Quadruple wrap the bottom and sides of the springform pan in heavy foil. We need the foil to keep the water out of the springform pan.
Pour the filling into the springform pan.
Place the pan in a large roasting pan and fill the roasting pan with boiling water, about 3/4" DO NOT LET THE WATER GO OVER THE FOIL
Bake for 60 minutes, or until the cheesecake is set (middle barely jiggles).
Remove from oven, and very carefully run a knife around the cake, and let cool completely.