In a medium bowl, whisk together the flour, cocoa powder, baking soda, and salt. Set this mixture aside for later.

In a large mixing bowl, use an electric mixer to cream the room temperature butter and granulated sugar together until light and fluffy, about 3-4 minutes.

Beat in the eggs one at a time, ensuring each egg is fully mixed in before adding the next. Stir in the vanilla extract for extra flavor.

Gradually add the dry ingredients to the wet mixture, mixing on low speed just until the dough comes together. Be careful not to overmix; a few lumps are okay. You can choose to lightly swirl in the marshmallow fluff now or wait until after chilling the dough.

Cover the dough and refrigerate for at least 1 hour. Chilling the dough makes it easier to handle and helps prevent the cookies from spreading too much while baking.

About 20 minutes before you’re ready to bake, preheat your oven to 350°F (175°C). Prepare two baking sheets by lining them with parchment paper or silicone baking mats.

Use a cookie scoop or a tablespoon to scoop out the chilled dough and roll it into balls. Place the balls on the prepared baking sheets, spacing them about 2 inches apart. Press a small indent into the center of each dough ball and spoon a teaspoon of marshmallow fluff into each indent. Use a toothpick to swirl the marshmallow into the cookie.

Bake in the preheated oven for 10-12 minutes, or until the cookies are set around the edges but still soft in the center. For extra gooeyness, you can add a bit more marshmallow fluff on top halfway through baking.

Allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely. This helps the marshmallow set slightly and makes the cookies easier to handle.