Combine the water, sugar, butter, sifted cocoa powder and soda in a medium saucepan; stir over heat, without boiling, until sugar dissolves. Bring to a boil then reduce heat, simmer, uncovered, 5 minutes. Transfer mixture to large bowl; cool to room temperature.
Add flour and eggs to bowl; beat with a balloon whisk until mixture is smooth. Pour mixture into a 8 inch greased round dish. Bake cake in moderate oven of 180 'C for about 50 minutes. Stand cake in baking dish for 10 minutes, before turning onto wire rack.
To prepare the Ganache, melt the Chocolate in the microwave and add sour cream to it. Gently mix it and let it set aside for 10 minutes. level the cake and generously slather the ganache on the top and level it with a spatula.