In a large mixing bowl, cream together the butter, granulated sugar, and brown sugar until light and fluffy. Add the eggs one at a time, mixing well after each addition. 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 ingredients, mixing until just combined. Fold in the chocolate chips.

Cover the dough with plastic wrap and chill in the refrigerator for at least 30 minutes to help the cookies hold their shape while baking.

Preheat the oven to 350°F (175°C). Line a baking sheet with parchment paper.

Using a cookie scoop, drop rounded tablespoons of dough onto the prepared baking sheet, spacing them about 2 inches apart.

Bake the cookies in the preheated oven for 10-12 minutes, or until the edges are set but the centers are still soft.

Remove the cookies from the oven and immediately press a marshmallow half into the center of each cookie. Return to the oven for an additional 1-2 minutes, allowing the marshmallows to soften.

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

Serve the cookies warm with a glass of milk or your favorite hot beverage. They are especially delicious when paired with a cup of hot cocoa.

Store any leftover cookies in an airtight container at room temperature for up to 5 days. For longer storage, you can freeze the cookies for up to 3 months.