Preheat the oven to 180˚C/160˚C fan/gas mark 4 and grease and line one 23cm square baking tin.
Sift the flour, sugar, cocoa powder, baking powder, salt and coffee powder into a mixing bowl.
In a separate bowl, beat the eggs, buttermilk, butter and vanilla extract. Gradually beat into the flour mixture until combined.
Transfer to the prepared tin and bake for 30-40 minutes until a skewer inserted into the centre comes out clean. Leave to cool in the tins for a few minutes, then transfer to a wire rack to cool completely.
When cool, use a serrated knife to carefully slice in half horizontally, then slice each half again horizontally to make four slices. (Or, if you prefer, just leave it at two slices.)
Heat the cream in a saucepan over a low heat and place the chocolate in a heatproof bowl. When small bubbles form at the edges of the pan, pour the cream over the chocolate and stir until melted.
Place the bottom layer of the sponge on your serving plate and spread over a very thin layer of ganache. Repeat these layers until the cake is assembled. Pour the remaining ganache onto the top of the cake and use a knife to push it to the edges of the cake (it should spill over the sides).
Drizzle the white chocolate over the top and decorate with grated dark chocolate.