First make the sponge. Preheat the oven to 170°( (fan 150°C)/325°F /gas mark 3. Grease 2 x 22cm (8½ inches) diameter shallow cake tins and line with baking paper.
Melt the chocolate in a heatproof bowl set over a saucepan of barely simmering water, making sure the surface of the water does not touch the bowl.
In a large bowl, cream the butter and sugar together until light and fluffy. Beat in the black treacle. Beat in the eggs, a little at a time, then add the melted chocolate. Sift the flour, cocoa powder and baking powder together, then fold in with the ground almonds. Finish with an electric hand whisk to produce a smooth, glossy consistency.
Divide the mixture between the prepared tins and level the tops. Bake in the oven for 30-35 minutes, or until a skewer inserted into the centres comes out clean. Leave to cool in the tins, then turn out on to a cooling rack.
To make the ganache, put the whipping cream into a saucepan and heat to just below boiling point. Remove from the heat and add the chocolate, beating vigorously until melted and smooth. Leave to cool to room temperature. To make the buttercream, melt the chocolate as above and leave to cool.
In a bowl, cream the icing sugar and butter together using an electric hand whisk until light and fluffy. Beat in the vanilla, cooled chocolate and double cream to form a fairly stiff spreading consistency.
To assemble the cake, level the tops of the sponges using a sharp knife.
Spread the buttercream over the top of one of the sponges and place it on a cooling rack set over a large plate or baking tray. Place the other sponge, upside down, on top to give a nice flat surface. Pour the ganache over the cake. Smooth with a palette knife and leave to set. Serve decorated with berries or dried fruits and nuts.