Preheat and prep: Heat the oven to 350°F (175°C).
Line two baking sheets with parchment paper. Set aside.
Mix dry ingredients: In a medium bowl, whisk 1 3/4 cups flour, 1/2 cup cocoa powder, 1 teaspoon baking soda, and 1/2 teaspoon salt until combined.
Combine wet ingredients: In a large bowl, whisk 1/2 cup melted unsalted butter (slightly cooled), 3/4 cup brown sugar, and 1/2 cup granulated sugar until glossy. Whisk in 2 large eggs, 1 teaspoon vanilla, and 1/2 teaspoon peppermint extract.
Bring the dough together: Stir the dry ingredients into the wet just until no dry spots remain.
The dough should be thick and a bit sticky. If it looks too soft, rest it 5 minutes to hydrate.
Fold-ins: Gently mix in 1 cup chocolate chips, 1/2 cup mini marshmallows, and 1/3 cup crushed peppermint. Reserve a little of each for topping.
Scoop: Use a medium cookie scoop (about 1 1/2 tablespoons) to portion dough onto prepared sheets, leaving 2 inches between.
Press a few extra chocolate chips and marshmallows on top for a bakery look.
Bake: Bake 8–10 minutes, until the edges are set and the centers look slightly underbaked. They will finish setting on the tray.
Finish and cool: Immediately sprinkle with a pinch of crushed peppermint and, if you like, a tiny bit of flaky sea salt. Cool on the sheet for 5 minutes, then move to a rack to cool completely.
Optional drizzle: Melt 1/3 cup white chocolate chips and drizzle over cooled cookies.
Add a final dusting of peppermint for sparkle.