01 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
02 - In a medium bowl, whisk together flour, cocoa powder, baking soda, salt, and espresso powder.
03 - In a large bowl, beat softened butter with granulated and brown sugars until creamy and light, about 2 minutes.
04 - Add egg, vanilla extract, and peppermint extract to the butter mixture. Beat until combined.
05 - Gradually add the dry ingredients to the wet mixture, mixing just until incorporated.
06 - Fold in semisweet chocolate chunks and half of the crushed peppermint candies.
07 - Scoop tablespoon-sized dough balls onto prepared sheets, spacing 2 inches apart.
08 - Bake for 9 to 11 minutes, until edges are firm but centers remain slightly soft.
09 - Remove from oven. While warm, sprinkle with remaining crushed peppermint candies. Let cool on sheets for 5 minutes, then transfer to wire rack.
10 - Once cookies are fully cooled, drizzle with melted white chocolate if desired. Allow chocolate to set before serving or storing.