Grease a 17 x 23 cm (7 x 9 inch) rectangular cake tin and line the base with baking paper. Put the chocolate and butter in a heatproof bowl set over a saucepan of gently simmering water (don't let the bowl touch the water) and stir over a low heat until melted. Stir in all the remaining ingredients.
Spoon the mixture into the prepared tin. Press well into the base and sides of the tin and smooth the surface with a palette knife.
Cover with foil and chill for 4 hours or overnight in the refrigerator. Carefully work round the edges of the cake with the palette knife and unmould on to a board, removing the paper from the base. Dust with icing sugar, if wished, and serve in thin fingers.