01 - Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper.
02 - In a large bowl, cream together the softened butter and packed brown sugar until light and fluffy. Beat in the large egg and vanilla extract until well combined.
03 - In a separate bowl, whisk together the all-purpose flour, unsweetened cocoa powder, baking soda, and salt. Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Fold in the chocolate chips.
04 - Divide the cookie dough into 12 portions. Flatten one portion of dough in your hand and place a mini marshmallow or a half of a large marshmallow in the center. Carefully enclose the marshmallow with the dough, ensuring it is completely sealed. Repeat with the remaining dough and marshmallows.
05 - Place the stuffed cookie dough balls onto the prepared baking sheet, leaving some space between them. Bake for 10 to 12 minutes, or until the edges are set but the centers are still slightly soft. Let the cookies cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely. Dust with powdered sugar before serving.