Mix the cocoa powder and water together until smooth.
Using the beater attachment, beat together the butter and white and brown sugar until light and fluffy.
Add the cocoa mix and beat together well.
Scrape down the bowl to make sure the cocoa is well mixed in.
Slowly add the beaten egg.
Fold in the flour until well combined.
Spoon the mixture into the prepared tin.
Bake for approximately 45-50minutes.
Prepare the frosting by melting together the chocolate and butter in a bowl over a pan of hot water. (Don’t let the water in the pan touch the bottom of the bowl) or melt together in a microwave.
Once melted remove the bowl from the pan of hot water and whisk in the cocoa and icing sugar.
Allow to cool then beat well to thicken the frosting before spreading on top of the cooled chocolate cake.
For a textured finish grate some more chocolate over the top.