Combine the water, sugar, butter, cocoa and soda bicarbonate in a medium sauce pan, stir over heat and bring to boil then simmer on low heat, uncovered for 5 minutes. Transfer the mixture to a large bowl and cool to room temperature.
Add flour and egg to the mixture and mix all together in an electric mixture until the mixture becomes smooth and changed to a paler color.
Grease a deep 26cmx33cm baking dish and cover with baking paper then pour the mixture in. Put the cake into a 180˚C preheated oven for about 45-50 minutes or until the cake becomes firm. Turn cake upside down to cool.
Combine NESTLÉ® Sweetened Condensed Milk , crushed chocolate and whipping cream in a bowl over hot water, stir until chocolate is melted. Cool to room temperature.
Spread the chocolate frosting on top of the cooled cake. Slice and serve.