Mix the crushed digestive biscuits and melted butter in a bowl, transfer this mixture to a baking sheet lined springform pan,spread well and press the mixture firmly,evenly.Keep in fridge for half an hour.
Meanwhile, melt the chocolate in double boiler method.
Beat the sugar and eggs in a bowl until the mixture turns light and creamy.
Add the cream cheese and beat for a while, add in the melted chocolate, cocoa powder and vanilla and whisk until the mixture gets well mixed.
Fold in the sour cream and mix well. Preheat the oven to 350F.
Pour this chocolate cream cheese mixture over the biscuit base, smooth the surface.
Bake for 45minutes or for an hour, let it cool completely.
Cover and place in fridge for 2 hours to chill.
Slice the cheesecake as you desire and enjoy.