1.Preheat oven to moderate. Grease deep 19cm-square cake pan, line base with baking paper.
2.Combine butter and chocolate in medium saucepan, stir over low heat until chocolate melts. Stir in caster sugar and egg then flour. Spread mixture into prepared pan, bake in moderate oven about 20 minutes. Stand cake in pan 15 minutes, turn onto wire rack to cool.
3.Meanwhile, combine icing sugar, extra butter and essence in small heatproof bowl, gradually stir in enough milk to make mixture form a thick paste. Stir mixture over small saucepan of simmering water until icing is of spreadable consistency. Spread icing over cake, allow to set at room temperature.
4.Using serrated knife, trim crisp edges from cake. Cut cake into 3cm squares, drizzle each square with melted extra chocolate.
Peppermint essence is an extract of distilled peppermint leavesNote