350 butter, softened
2 cups icing sugar
3 egg yolks
3 1/2 cups flour
3 tablespoons peppermint essence
2 teaspoons red food colouring
200g dark chocolate
Beat butter and 1 1/2 cups of the sugar in a large bowl with an electric mixer on low speed. Increase speed to medium and beat until light and fluffy. Add yolks one at a time, beating until smooth after each addition. Mix in 2 tablespoons of the peppermint essence, then reduce speed to low and add flour 1/2 cup at a time, beating after each addition to combine. Remove half the mixture and set aside. Add red food colouring to remaining mixture and mix well.
Place one dough ball between 2 pieces of baking paper. Roll dough out to large rectangle about 3mm thick. Repeat with other dough. Wrap in cling film and place on a plate and refrigerate for 1 hour.
Preheat oven to 160°C.
Remove dough from fridge. Layer one colour on top of the other, brushing a little water between them and leave for a few minutes to fuse and come to room temperature, then cut sheet in half. With the flat edge to the top, roll each sheet tightly into a cylinder. If the dough cracks leave it to warm a bit longer or smooth the cracks over with a little water.
Wrap log in cling film and place in freezer for 15 minutes to firm.
Remove from freezer and slice thinly, about 3mm thick. Place rounds onto a baking tray lined with baking paper.
Bake for about 15 minutes, until edges just start to turn golden. Transfer to a rack to cool completely.
If you’re feeling it, you can combine chocolate and remaining peppermint essence in a heatproof bowl over a saucepan of boiling water to melt, and dunk the biscuits in the chocolate, for an extra after dinner mint edge.