Preheat oven to 350°F. Line a baking sheet with parchment paper. Gather all ingredients and equipment.

In a large mixing bowl, beat together the butter, brown sugar, and granulated sugar until creamy. Add the eggs one at a time, mixing well after each addition. Stir in 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 prevent spreading during baking.

Scoop tablespoons of dough onto the prepared baking sheet, spacing them 2 inches apart.

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

While cookies are still warm, press a marshmallow half into the center of each cookie. Allow the heat to slightly melt the marshmallow.

Let cookies cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely. Serve warm for the best gooey experience.

Store cooled cookies in an airtight container at room temperature for up to 5 days. To freeze, place cookies in a single layer on a baking sheet and freeze until solid, then transfer to a freezer bag for up to 3 months.