In a deep bowl mix the sugar and butter until frothy. Add the eggs one by one, whisking between each addition. In another bowl, put the water, cocoa and instant coffee to combine. Add the sour cream and milk to the mixture and stir. Add the vanilla extract. Mix the flour, baking powder and salt together. Add 1/3 of the flour and 1/3 of the liquid ingredients with cocoa to the butter and egg mixture, stir and repeat the procedure twice more, adding the dry and then the liquid ingredients. Pour the mixture in a greased muffin tray and bake in a preheated oven at 160 degrees for about 15 minutes. Melt the chocolate, butter and milk together in a steam bowl and allow to cool slightly. Turn the muffins over, cut the raised pieces so that they can stand flat, and sprinkle them with the chocolate mixture. Decorate with whipped cream.