Preheat the oven to 180C/160C Fan/Gas 4. Grease the tin with margarine and line the base and sides with baking paper. You need to line the tin right to the top even though the sponge will not fill it.
For the chocolate cake, measure the cocoa powder into a large bowl. Pour over the boiling water and mix to a paste with a spatula. Add the rest of the dry ingredients and the eggs and margarine and beat until smooth using a hand-held mixer.
Spoon the cake mixture into the prepared cake tin and level the surface with a palette knife. Bake in the oven for 20-25 minutes, or until a skewer inserted into the centre of the cake comes out clean and the cake feels springy to the touch.
While the cake is still hot, brush the brandy over the top of the cake. Leave the cake to cool in the tin.
Meanwhile, for the mousse, place the chocolate in a bowl and melt over a pan of gently simmering water (do not allow the base of the bowl to touch the water). Stir continuously, taking care not to let the chocolate get too hot. Set aside to cool a little.
Whip the cream until soft peaks form when the whisk is removed. Carefully fold in the melted chocolate until smooth and not streaky.
When the cake has cooled, and while it is still in the tin, spoon the chocolate mousse on top and level with a palette knife. Cover the cake tin with cling film and chill in the fridge for a minimum of 4 hours, and preferably overnight, until the mousse is firm.
To serve, carefully remove the cake from the tin and transfer it to a flat plate. Dust the top with cocoa powder, then pile the raspberries and blueberries into the centre. Finish with a light dusting of icing sugar. Cut into wedges and serve with pouring cream.
Tip 1: Use chocolate with no more than 40–50 per cent cocoa solids as the richer types of chocolate can make this dessert too bitter.
Tip 2: This dish can be made 1-2 days in advance - just chill in the fridge without the fruit topping until needed. The sponge base can be made ahead and frozen, then defrosted prior to adding the mousse layer.