1. Beat the softened butter and caster sugar together for 2–3 minutes until smooth and creamy. You don’t need to incorporate lots of air as you would for a cake.
2. Add the vanilla and egg yolk. Beat in the vanilla, followed by the egg yolk, until fully incorporated. Scrape down the sides of the bowl.
3. In a separate bowl, whisk together the flour, cocoa powder, cornflour and salt.Make the dough.
4. Add the dry ingredients to the butter mixture and mix on low speed just until the dough starts to come together. Avoid overmixing, if it looks a little crumbly initially, use your hands to gently bring it together. Wrap in cling film and chill.
5. Divide the dough in half. Roll each portion between two sheets of baking paper to approximately 4mm thick.
6. Remove the top sheet of baking paper and cut into 5–6cm rounds for regular sandwich cookies, or 3–4cm rounds for minis.
7. Transfer to lined baking trays, leaving a little space between each cookie. Re-roll the scraps once and cut again.
8. Bake at 170°C fan-forced for 9–11 minutes for regular cookies or 7–9 minutes for minis.
9. Cool on the baking tray for 5 minutes before transferring carefully to a wire rack. Cool completely before filling with the white chocolate mint ganache.

White Chocolate Mint Ganache

1. Place the finely chopped white chocolate in a heatproof bowl.
2. Heat the cream in a small saucepan over medium-low heat until just steaming, don’t let it boil.
3. Pour the hot cream over the white chocolate and leave undisturbed for a few minutes.
4. Stir gently from the centre outwards until completely smooth, add the flavouring. Stir in the butter, ⅛ teaspoon peppermint extract and a small pinch of salt until glossy and combined. Chill
5. Refrigerate for 30–45 minutes, stirring occasionally, until thick enough to pipe or spread. Don’t let it become completely firm.

Assemble Cookies

1. Match the cookies. Pair the cooled cookies by size and shape so the sandwiches sit neatly together.
2. Spread the white chocolate mint ganache over the flat side of one cookie, leaving a small border around the edge. Top with a second cookie and press gently until the filling reaches almost to the edge.
3. Place the assembled cookies in the refrigerator for 15–20 minutes to firm the ganache before dipping.
4. Melt the dark chocolate gently in a heatproof bowl over simmering water or in short bursts in the microwave. Dip half of each sandwich cookie into the chocolate, allowing the excess to drip back into the bowl.
5. Add the peppermint coating. While the chocolate is still wet, sprinkle the dipped portion with finely crushed peppermint candy/candy canes.
6. I’d keep this fairly restrained—the cookie already has mint in the ganache and you don’t want the peppermint crunch to overwhelm the dark chocolate.
7. Place on a baking-paper-lined tray and leave until the chocolate has completely set. For a faster set, refrigerate for around 10 minutes.
If you want an easier method to decorate, simply drizzle white or dark or both chocolates over the finished cookies and sprinkle the crushed peppermint over the top.