Preheat oven to 350 degrees F. Melt semi-sweet chocolate in double boiler and then let cool. In a bowl beat the cream cheese until smooth. Then beat in sugar, the eggs, 1 at a time, melted chocolate, cocoa, and vanilla. With a rubber spatula fold in the sour cream until thoroughly mixed. Pour into pan and bake 50-60 minutes or more. Cool at room temperature for 1 hour and then refrigerate for 3-4 hours.