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

In a large mixing bowl, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy.

Add the eggs one at a time, mixing well after each addition, then stir in the vanilla extract.

In a separate bowl, whisk together the flour, cocoa powder, baking soda, and salt.

Gradually add the dry ingredients to the wet mixture, mixing on low speed until just combined.

Fold in the chocolate chips and marshmallows until evenly distributed within the dough.

Scoop rounded balls of dough onto the prepared baking sheets, leaving about 2 inches of space between each cookie.

Bake for about 10-12 minutes, until the edges are set but centers remain soft.

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

Optionally, dust cooled cookies with powdered sugar before serving.