Melt the chocolate in a heatproof bowl set over a pan of barely simmering water. Line a large baking tray with baking parchment and mix the sugar with the peppermint essence. When the chocolate has melted, cool for 1 min, mix with the sugar, then pour into the tray. Spread to a very thin layer and chill until completely firm. Snap into shards to serve. Perfect as a garnish for puddings or with an after-dinner coffee.