01 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
02 - Whisk together flour, cocoa powder, baking soda, and salt in a medium bowl.
03 - Beat butter, granulated sugar, and brown sugar together in a large bowl until light and fluffy, about 2 to 3 minutes.
04 - Incorporate egg, vanilla extract, and peppermint extract into the butter mixture; beat until well combined.
05 - Gradually add the dry ingredients to the wet mixture, stirring just until combined.
06 - Fold in chocolate chips and half of the crushed peppermint candies.
07 - Drop tablespoon-sized portions of dough onto the prepared baking sheets, spacing them approximately 2 inches apart.
08 - Bake cookies for 8 minutes, then gently press several mini marshmallows onto each cookie top. Return to oven and bake an additional 2 minutes until cookies are set and marshmallows slightly melt.
09 - Remove cookies from oven and immediately sprinkle remaining crushed peppermint candies on top. Cool on baking sheets for 5 minutes before transferring to wire racks to cool completely.