Preheat your oven to 350°F (175°C) and line baking sheets with parchment paper.
In a large mixing bowl, beat softened butter, granulated sugar, and brown sugar using an electric mixer until light, fluffy, and pale, about 3 to 4 minutes.
Add eggs one at a time, mixing well after each addition, then stir in vanilla extract.
In a separate bowl, whisk together all-purpose flour, unsweetened cocoa powder, baking soda, and salt until combined.
Gradually add dry ingredients to the wet ingredients, mixing on low speed or folding gently until just combined.
Fold in chocolate chips and mini marshmallows evenly.
Scoop dough onto prepared baking sheets about 2 inches apart. Bake for 10 to 12 minutes until edges are set but centers are still soft.
Let cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.