Preheat the oven to 180°C (160°C Fan, Gas Mark 4). Lightly grease 2 x deep 18cm (7 inch) Victoria sandwich tins.
Break the Extra Dark Chocolate into pieces and place in a large heatproof bowl. Put the bowl over a saucepan of barely simmering water and leave to melt. Remove from the water.
Meanwhile, put the flour, sugar and Bicarbonate of Soda in a large bowl. Mix together and make a well in the centre.
When the Chocolate has melted, scrape into the well and add the butter, eggs and beer. Carefully mix everything together then whisk for a few seconds until thoroughly blended.
Divide the mixture between the prepared tins and smooth the tops. Put the tins on baking trays and bake in the oven for 25-30 minutes until risen and firm to the touch. Cool for 10 minutes before removing from the tins and transferring to a wire rack to cool completely. Carefully slice through the middle of each sponge to create 4 equal cake slices.
For the filling, break up the Extra Dark Chocolate into a large heatproof bowl and melt as above. Remove from the water and leave to cool for about 10 minutes until warm but not hot.
Whilst whisking the Chocolate, gradually pour in the cream, whisking until thickly whipped and glossy. Use the filling to sandwich the 4 sponges together and place on a wire rack over a board.
To decorate, break the Extra Dark Chocolate into a small, heavy bottomed saucepan. Add the butter and pour in 20ml (4 tsp) water. Heat gently, stirring occasionally, until melted together and smooth. Remove from the heat and set aside for about 20 minutes to cool to a thick, coating consistency.
When the Chocolate starts to thicken, spoon the ganache over the top of the cake, allowing it to drip over the edges. Use a small bladed palette knife to smooth over any gaps and leave to stand in a cool place for several minutes to allow the Chocolate to set.
Carefully, slide 2 palette knives or cake slices, one on each side of the cake, to loosen and then lift the cake on to a serving plate.
