Preheat your oven to 350°F (175°C), and prepare two baking sheets by lining them with parchment paper for easy cleanup.
In a large mixing bowl, beat together the softened butter, granulated sugar, and packed brown sugar using an electric mixer until the mixture is creamy, light, and fluffy, about 3-4 minutes.
Add the eggs one at a time, ensuring each is fully incorporated before adding the next. Follow by mixing in the vanilla extract until well combined.
In a separate bowl, sift together the all-purpose flour, unsweetened cocoa powder, baking soda, and salt. Gradually incorporate this dry mixture into the wet ingredients, mixing on low speed until just combined. Be careful not to overmix.
Gently fold in the semi-sweet chocolate chips and mini marshmallows using a spatula, ensuring they are evenly distributed throughout the cookie dough.
Using either a spoon or a cookie scoop, place heaping tablespoonfuls of dough onto the prepared baking sheets, spacing each mound about 2 inches apart to allow for spreading.
Take small dollops of the marshmallow fluff and place one on top of each cookie dough mound for an enticing swirl.
With a toothpick or a small knife, carefully swirl the marshmallow fluff into the cookie dough to create a delightful marbled effect, ensuring not to mix too thoroughly.
Bake in your preheated oven for 10-12 minutes. You'll know they're done when the edges are set and tops are slightly cracked—remember that the cookies will appear soft but will firm up as they cool.
Once baked, remove them from the oven and allow the cookies to cool on the baking sheet for about 5 minutes. Afterward, transfer them to wire racks to cool completely.
Notes
Arrange the cookies playfully on a serving platter, and if desired, drizzle with melted chocolate or dust with powdered sugar for an extra touch of sweetness and visual appeal. Enjoy them with a refreshing glass of milk!