Preheat the oven to 180°C, Fan 160°C, Gas Mark 4. Lightly grease 2 deep 7" diameter cake tins.
Break the Extra Dark Chocolate into pieces and place in a large heatproof bowl over a saucepan of barely simmering water and leave to melt. Remove from the heat.
Meanwhile put the flour, sugar and bicarbonate of soda in a large bowl. Mix together and make a well in the centre.
Scrape melted chocolate into the well and add the butter, eggs and beer. Carefully mix together then whisk until thoroughly blended.
Divide mixture between tins and smooth. Put on baking trays and bake for 25-30 minutes until risen and firm to touch. Cool for 10 minutes before removing from tins and transferring to a wire rack to cool completely. Carefully slice each sponge into 2, to create 4 equal cake layers.
For the filling, break 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.
Whisk the chocolate while gradually pouring in the cream, whisking until thick and glossy. Use to sandwich the 4 sponges together. Place cake 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 it starts to thicken, spoon the glaze over the top of the cake, allowing it to drip over the edges. Use a small bladed palette knife to smooth any gaps and leave in a cool place for several minutes to set.
Carefully slide 2 palette knives or cake slices under the cake, one on each side, to loosen and then lift it on to a serving plate.