One 23 cm (9 inch) square cake (we used mud cake, but you could use chocolate cake)
1 sheet of cardboard for the stencil
dark cocoa powder, to dust
chocolate sticks, to decorate
Dark chocolate buttercream
90 g (3 oz) dark chocolate,
chopped 150 g (5 oz) unsalted butter
2 tablespoons icing sugar
1 - Put the cake on the piece of cardboard and draw around the outside to make an outline. Using a serrated knife, cut the domed top off the cake to leave a flat surface. If you want, trim the sides of the cake to make a very even, sharp-edged square. Turn the cake upside down on a cake board or serving plate, so that the flat base of the cake becomes the top.
2 - Draw a design on the cardboard and cut out with a scalpel or sharp scissors. Try to use a firm piece of board so the stencil won't be too floppy.
3 - To make the buttercream, bring a pan containing a little water to a simmer. Put the chocolate in a heatproof bowl, remove the pan from the heat and place the bowl over the pan (don't let the base of the bowl sit in the water). Stir the chocolate until melted. Beat the butter with electric beaters until light and creamy. Gradually beat in the cooled melted chocolate and icing sugar and continue beating until pale and creamy. Spread the buttercream evenly over the top and sides of the cake, smoothing the surface as cleanly as possible with a palette knife. Refrigerate for 30 minutes, or until the butter¬cream is firm.
4 - Gently place the stencil over the top of the cake. Place a small amount of cocoa powder in a fine sieve and shake lightly over the stencil. Carefully lift the stencil off the cake without spilling any cocoa that is still on the cardboard. Press the chocolate sticks, upright, around the side of the cake—if they are too long, or uneven, trim them all so they stand just a little higher than the top of the cake.
Cut the cardboard to the same size as the cake, draw on a design and cut out.
Spread the buttercream evenly over the top and sides of the cake.
Place the stencil over the top of the cake and shake the cocoa powder over it.
Ahead of time: This cake can be decorated up to 2 days in advance. Store in the fridge in warm weather, or in an airtight container in a cool, dry place.