To make crust, combine the wafer crumbs, 2 tablespoons melted butter and 1 tablespoon sugar. Press into bottom of a 23cm springform cake tin. Wrap the tin bottom with aluminium foil. Refrigerate while preparing the filling.
To make filling, grate the chocolate and put it into the top of a double boiler. Place over hot water. When almost melted, remove from heat and stir until completely melted. Chocolate can also be melted in a microwave, stirring frequently.
Combine the cream cheese and the sugar; mix with an electric mixer until well blended.
Dissolve the coffee powder in the coffee liqueur. Add the coffee mixture and the melted chocolate to the cream cheese mixture. Blend well.
Add the eggs one at a time, mixing until just blended after each egg.
Pour the filling into the crust, smoothing the top.
Place the cake tin in a large roasting tin and pour boiling water into the roasting tin until it comes halfway up the sides of the cake tin.
Bake in a preheated 180 degree C oven for 1 hour.
Remove the cheesecake from the oven and remove the cake tin from the water bath.
To make topping, stir together the sour cream, 2 tablespoons sugar and 1 teaspoon coffee liqueur. Spread over the top of the hot cheesecake. Put back in the oven for 5 minutes.
Refrigerate at least 4 hours, if possible overnight.