Place your demerara sugar in a small bowl. Sprinkle peppermint extract over the top of the sugar and stir well.
Melt the dark chocolate.
To get a good snap in your chocolate mints it's best to temper your chocolate. I use Callebaut dark chocolate callets and melt them in the microwave in 30 second bursts - this will take 3 or 4 bursts depending on your microwave. Melt and stir until your chocolate is fluid and at a temperature of 31° C to 32° C. Before your chocolate is fully melted you will still see a few whole callets/drops as you stir - this shows you are almost there. Simply keep stirring until the callets/drops have melted.
Have your piece of baking parchment ready.
Add your peppermint sugar to the chocolate and stir to mix.
You need to work quickly at this stage as the chocolate is cooling and thickening as you add the sugar.
Pour the mixture onto the baking parchment and spread the chocolate mix with a palette knife. You don't need to worry about getting too smooth a surface.
Leave to set for about 5 to 10 minutes - the timing depends how warm your kitchen is.
Using a large sharp knife, cut into rectangles of about 2" x 1". If you prefer, cut into shards.
Leave to set completely. If you don't have a cool place to leave your chocolate peppermint crisp, leave it to set in the fridge.
Once it has set, carefully separate your chocolate peppermint crisps.
Store in an airtight container.