Preheat the oven to 180°C, 170°C fan, Mk 4. Put some water in a cake tin or roasting tin and put in the base of the oven. Line the tins with baking parchment.
In a small pan set over low heat melt together the chocolate and butter with 100ml water.
Sift together the flour, Billington’s sugar, cocoa powder and baking powder. Mix in the eggs until well blended. Stir in the chocolate and butter. Spoon into the prepared tins and smooth the top. Bake for 30 minutes, until a skewer comes out clean when inserted. Remove from the tin, and set aside on a wire racks to cool.
Meanwhile, make the filling and icing. For the filling beat together all of the ingredients and refrigerate until required. For the icing, mix together the Billington’s icing sugar, cocoa and dried egg white, then stir in the water until a dropping consistency. You may need a little more water.
To assemble, put a dab of buttercream filling on the centre of the serving plate, and put one cake on the plate. Spoon on the remaining filling, and top with the other cake. Spoon over the icing –to encourage it to drizzle evenly, spoon into the middle, and with the back of the spoon, make a circular motion on top of the cake. Allow the icing to dry for 10 minutes before decorating.