320 gm dark chocolate (70% cocoa solids), finely chopped320 gm (2 cups) pure icing sugar, sieved1 tbspvegetable oil2-3 dropspeppermint extract, or to taste
Line a 21cm-square cake tin with baking paper. Melt half the chocolate in a heatproof bowl over a saucepan of simmering water, stirring occasionally, until smooth. Pour into prepared tin and tilt to cover the base evenly, then tap firmly a few times on a work surface to level it and remove any air bubbles. Freeze until hardened (4-5 minutes).
Combine icing sugar, oil, peppermint extract and 1½ tbsp water in a bowl and mix to a smooth paste, then warm gently over a saucepan of simmering water until easily spreadable (1-2 minutes). Spread evenly over chocolate.
Melt remaining chocolate, pour onto peppermint mixture, tilt to cover evenly, then tap firmly a few times on a work surface to level it and remove any air bubbles. Refrigerate until set (15-20 minutes).
To cut, dip a sharp knife into a large jug of just-boiled water, wipe dry and carefully cut into 5cm squares, wiping the blade of the knife clean between cuts and re-dipping in hot water if the blade cools down; or break it into rough shards. Refrigerate in an airtight container until required. After-dinner mints will keep for 2 weeks.