Preheat your oven to 350°F (175°C) and prepare baking sheets lined with silicone mats or parchment paper.

In a stand mixer, combine softened butter and sugar, beating until light and fluffy for about 2-3 minutes.

Add eggs one at a time to the butter and sugar mixture, mixing well after each addition until smooth.

In a separate bowl, whisk together flour, cocoa powder, baking soda, and salt; then gradually add to wet ingredients, mixing on low speed until combined.

Fold in mini marshmallows and chocolate chunks using a spatula, being careful not to overmix.

Scoop equal-sized portions of dough onto the baking sheets, leaving space for spreading.

Bake for 10-12 minutes until the edges are set and centers are slightly underbaked.

Allow cookies to cool on the baking sheet for about 5 minutes before transferring to wire racks.