01 - Set 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 in a large bowl until light and fluffy, about 2 to 3 minutes.
04 - Add egg, vanilla extract, and peppermint extract to the butter mixture and mix until combined.
05 - Gradually mix dry ingredients into the wet ingredients just until incorporated.
06 - Fold in chocolate chips and half of the crushed peppermint candies.
07 - Scoop tablespoon-sized dough portions onto prepared baking sheets, spacing about 2 inches apart.
08 - Bake for 8 minutes, then gently press mini marshmallows onto each cookie.
09 - Return cookies to oven and bake an additional 2 minutes or until set and marshmallows soften.
10 - Remove from oven, immediately sprinkle with remaining crushed peppermint candies. Let cool on baking sheets for 5 minutes before transferring to wire racks to cool completely.