Preheat the oven to 180°C and grease a 20cm tin with margarine and baking paper.
Mix cocoa powder and boiling water into a paste in a large bowl.
Add the remaining cake ingredients and eggs to the bowl and beat until smooth.
Spoon the mixture into the tin and bake for 20 to 25 minutes.
Brush brandy over the hot cake and let it cool completely in the tin.
Melt the chocolate in a bowl over a pan of simmering water and let it cool slightly.
Whip the cream to soft peaks and fold in the melted chocolate.
Spread the mousse over the cooled cake and chill for at least 4 hours.
Remove from tin and garnish with cocoa powder, fresh berries, and icing sugar.