Grease 19cm x 29cm slice pan, line base and two long sides with baking paper extending paper 2cm over long sides.
Process the biscuits until fine.
Combine butter and milk in a small saucepan; stir over low heat until smooth. Combine processed biscuits with chocolate bar in a medium bowl; stir in butter mixture. Press mixture firmly into pan; refrigerate, covered, about 20 minutes or until set.
Meanwhile, stir ingredients for chocolate topping in small heatproof bowl over small saucepan of simmering water, until smooth; spread mixture over slice. Refrigerate until firm before cutting into 24 squares.
VARIATION
Lemon Slice
Replace Peppermint Crisp bars with 1 teaspoon finely grated lemon rind and 1 tablespoon lemon juice in the biscuit mixture. Press mixture firmly into pan; refrigerate covered, about 20 minutes or until set. Top with lemon icing made by stirring 400g icing sugar with 20g butter and two tablespoons lemon juice in small heatproof bowl over simmering water until smooth.