Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.

In a stand mixer, combine softened unsalted butter and brown sugar, beat on medium speed until light and fluffy, about 3-5 minutes.

Add two large eggs and a splash of vanilla extract to the creamed mixture, mixing on low speed until well combined, about 1-2 minutes.

In a separate bowl, whisk together flour, cocoa powder, baking soda, and salt. Gradually add to the wet mixture, mixing on low until just combined.

Fold in chocolate chips and mini marshmallows with a spatula, ensuring even distribution.

Using a cookie scoop, portion out equal amounts of dough onto the prepared baking sheet, leaving space for spreading.

Bake in the preheated oven for 10-12 minutes, until the edges are crisp but the centers are soft.

Let the cookies cool on the baking sheet for about 5 minutes before transferring to a wire rack to cool completely.