1. Cream butter, brown sugar, and granulated sugar until light and fluffy.

2. Add egg and vanilla extract; mix well.

3. In a separate bowl, whisk flour, cocoa powder, baking soda, and salt.

4. Gradually mix dry ingredients into the wet mixture to form a dough.

5. Chill dough in the fridge for 30 minutes.

6. Scoop tablespoon-sized balls of dough onto parchment-lined baking sheets.

7. Bake at 350°F (175°C) for 8–10 minutes. Cool completely.

8. For the filling, beat butter and powdered sugar until smooth.

9. Add peppermint extract and heavy cream; beat until fluffy and spreadable.

10. Spread filling onto half the cookies, then top with remaining cookies.

11. Optional: roll edges in crushed peppermint candies.

12. Store in an airtight container for up to 5 days or freeze for a month.