1. In a large bowl, cream together the unsalted butter, brown sugar, and granulated sugar until light and fluffy. Beat in the eggs one at a time, then mix in vanilla extract. In a separate bowl, whisk together the flour, cocoa powder, baking soda, and salt. Gradually combine with the wet ingredients. Fold in chocolate chunks until evenly distributed. Scoop out cookie dough, flatten slightly, and stuff 3–4 mini marshmallows into the center. Roll to seal. Chill the dough balls in the refrigerator for at least 30 minutes. Preheat oven to 350°F (175°C). Place dough balls on parchment-lined baking sheet. Bake for 9–11 minutes, until edges are set and centers are soft. Let cool slightly before transferring to a wire rack. Serve warm for best gooey texture.