1. Combine the flour, cocoa powder, baking soda, and salt. In a separate bowl, cream the butter and sugar until light and fluffy. Beat in the egg and vanilla. Gradually mix in the dry ingredients until just combined.

2. Wrap the dough and chill for at least 30 minutes. Preheat oven to 350°F (175°C). Roll dough into 1-inch balls and place on a lined baking sheet. Use a 1/2-teaspoon measure to gently press an indentation into the center of each cookie.

3. Bake for 9–11 minutes. Immediately upon removal from the oven, re-press the indentation. Cool completely on the baking sheet.

4. Place chocolate in a bowl. Heat heavy cream until simmering, then pour over chocolate. Let stand 5 minutes, then whisk until smooth. Stir in the peppermint extract. Let cool and thicken for 30–60 minutes.

5. Fill the cooled cookie cavities with the thickened peppermint ganache. Immediately sprinkle the tops with crushed candy canes. Allow the ganache to fully set before serving.