Begin by melting the butter in a medium bowl, then whisk in the granulated and brown sugars until well combined and slightly glossy. Beat in the eggs one at a time, followed by vanilla extract, mixing until smooth and creamy.

Gradually mix in the all-purpose flour, cocoa powder, and a pinch of salt, stirring just until combined.

Once the dough is chilled, preheat your oven to 350°F (175°C). Scoop tablespoon-sized portions of dough and roll them into balls. Press a mini marshmallow into the center of each ball.

Arrange your dough balls on a lined baking sheet, leaving space between each. Bake the cookies in the preheated oven for 10-12 minutes.

Remove the cookies from the oven and let them cool on the baking sheet for at least 5 minutes.