Makes about 2 1/2 dozen cookies.
Peppermint Filling and Assembly
For the cookie layer, stir the cookie crumbs, cocoa powder and salt together in a bowl. Add the condensed milk, butter, vanilla and peppermint extract and stir until evenly combined – the mixture will be very soft. Scrape this onto piece of plastic wrap and shape into a disc. Chill for at least 2 hours.
For the peppermint filling, place the condensed milk in a large bowl and use electric beaters (or do this in the bowl of a stand mixer fitted with the paddle attachment) to beat in the icing sugar a cup (130 g) at a time, scraping the bowl when needed – the mixture will come together like a soft dough. Beat in the peppermint extract and shape this into a disc, wrap in plastic wrap and chill for at least 2 hours.
Using icing sugar to dust your work surface, and roll out the peppermint filling to a square about 6-inches (15 cm) across and just over 1/4 -inch (6 mm) thick and set this aside Divide the chocolate dough into 2 pieces. Roll out the first piece of dough to the same thickness and repeat with the second piece of chocolate dough (the chocolate layers will be a little thinner than the peppermint one. Brush one side of the peppermint filling with a little bit of water and place this side on top of one piece of chocolate dough. Brush the opposite side with water and place the second chocolate piece on top. Gently roll it out a little to get the layers to adhere. Using a 1 1/2” (3.5 cm) round cutter, cut into rounds (if it is soft, pop this into the fridge for an hour or two, to set up) and place on a parchment lined baking tray. Alternatively, to avoid scraps, you can cut these into squares. Chill the patties while preparing the chocolate.
Melt the chocolate chips with the butter and corn syrup in a bowl placed over a pot of barely simmering water, stirring gently until smooth. Dip each cookie into the chocolate and shake off the excess before placing onto the tray to set. Melt the white chocolate in the same manner and using a parchment cone drizzle it over the cookies. Once set, the cookies can be packed in an airtight container and stored, refrigerated, for up to 2 weeks.