Pulse together the flour, cornflour, salt, icing sugar and butter in a food processor until it comes comes together to form a dough in the machine.
Remove the dough, knead gently and form into a ball. Wrap in clingfilm and chill for 60 minutes.
Preheat the oven to 180°C.
Remove from the fridge and roll out on a lightly floured surface to 1cm thickness. Cut out shapes from the dough using an egg-shaped cookie cutter. Grease and line 2 baking trays. Arrange on the baking trays spaced apart then bake for 12-15 minutes until they just start to colour. Remove from the oven and leave them to cool on their trays for a few minutes.
Transfer to wire racks to cool completely.
Place the two kinds of chocolate in separate heatproof bowls. Microwave in 30 second bursts, stirring in between, until you have smooth melted chocolate. Allow the chocolate to cool and thicken a little.
Using a clean paintbrush or thin pastry brush, paint the melted milk chocolate onto the top halves of half the cookies. Use the melted dark chocolate on the other half of the cookies. Transfer to wire racks and allow to cool and set before serving.