Preheat the oven to 350 degrees F (180 degrees C). Line the cake pan with parchment paper and brush or spray with oil. Cream together the butter and sugar in a large bowl until they begin to go pale. Add half the eggs and the all-purpose flour and mix well. Add the rest of the eggs, cake flour, unsweetened cocoa, salt, and baking powder and beat for a minute or two until the mixture is uniform. Dollop into the prepared pan and bake in the oven for 30 to 40 minutes, or until a skewer inserted in the middle comes out clean. Let cool in the pan.
Meanwhile, make the buttercream. Put the butter and confectioners' sugar in a bowl and whisk together until the mixture begins to go fluffy. Add the cooled melted chocolate and whisk for a further 2 minutes.
Once the cake is completely cool, remove from the pan. Carefully cut the top flat with a large serrated knife. (Eat this bit as a chef's perk.) Turn the cake upside down on an 8in (20cm) cake board so that the bottom now becomes a nice flat top. Split the cake horizontally and sandwich the top and bottom together with a 1/2in (1cm) layer of buttercream. Spread half the remaining buttercream all over the top and sides of the cake, making it as smooth as possible. Put it in the refrigerator to set before doing another layer. This makes it much easier to get good squared-off edges.
Gently push the cigarillos or wafers vertically onto the sides of the cake, positioning them as straight as possible and making sure they touch the bottom. The next step is up to you. I can't tell you the wide-eyed looks you'll get when you walk into the room holding the finished cake. Decorate with fresh flowers for a Christening or wedding, strawberries for a big family gathering, or with figs for a man's birthday. Serve with a self-satisfied grin.
This cake is made with chocolate cigarillos, but if you can't get them, use chocolate rolled wafers instead.