Preheat the oven to 160 degrees C. Grease a 23cm springform cake tin. Wrap the bottom of the tin in foil to prevent leaks.
In a double boiler or microwave melt the chocolate with the butter. Add the instant coffee, vanilla and salt. Beat in the sugar, then the eggs one at a time. Stir in the flour and pour the batter into the cake tin. Bake 30 minutes and let cool while you make the filling.
With an electric beater, beat the cream cheese until smooth, gradually adding the sugar. Add vanilla, instant coffee, salt, sour cream and cornflour and beat well. Add the eggs one at a time.
Pour the batter over the cooked base and place in a roasting tin. Pour in hot water to come 3cm up the sides of the cake tin. Bake for 70 minutes, then remove from the water bath and let cool on a rack.