1. Line 2 baking sheets with parchment paper and set aside.

2. In a large bowl, beat the softened butter and granulated sugar together until light and creamy, 2–3 minutes.

3. Add the egg yolk, peppermint extract, and vanilla extract, and beat until smooth and well combined.

4. In a separate bowl, whisk together the flour, cocoa powder, baking powder, and fine sea salt until no lumps remain.

5. Add the dry ingredients to the butter mixture in two additions, mixing on low speed until a soft dough forms.

6. Turn the dough out onto parchment paper and shape it into a log about 1 1/2 inches in diameter; wrap tightly.

7. Chill the dough in the refrigerator for at least 1 hour, or freeze for 20–30 minutes, until very firm.

8. Preheat the oven to 350°F (175°C). Slice the chilled dough into 1/4-inch rounds and place on prepared baking sheets.

9. Bake for 8–10 minutes, until the tops look dry and the edges are set; cool on the baking sheet for 5 minutes.

10. Transfer cookies to a wire rack and let cool completely before dipping.

11. For the coating, melt the chocolate and neutral oil or coconut oil together until smooth, using a double boiler or microwave.

12. Stir in the peppermint extract for the coating, tasting and adjusting the amount as needed.

13. Dip each cooled cookie into the melted chocolate with a fork, tapping off excess and placing on parchment to set.

14. Let the cookies stand at room temperature until the chocolate is firm, or refrigerate for 15–20 minutes to speed setting.

15. Store finished Copycat Thin Mints in an airtight container in the refrigerator or freezer until ready to serve.