Mix Dry Ingredients: In a medium bowl, whisk together 2 cups all-purpose flour, ¾ cup unsweetened cocoa powder, ½ teaspoon baking soda, and ¼ teaspoon salt. Set this mixture aside.

Cream Butter and Sugar: In a large mixing bowl, use an electric mixer to cream 1 cup of room temperature unsalted butter with 1½ cups granulated sugar until the mixture is light and fluffy, which usually takes about 3-4 minutes.

Add Eggs and Vanilla: Beat in 2 large room-temperature eggs one at a time, ensuring each is fully incorporated before adding the next. Stir in 2 teaspoons of vanilla extract.

Combine Wet and Dry: Gradually add the dry ingredients to the wet mixture, mixing on low speed just until the dough comes together. Avoid overmixing the batter.

Chill the Dough: Cover the dough and refrigerate for at least 1 hour. This makes the dough easier to handle and prevents the cookies from spreading too much in the oven.

Preheat and Prepare: About 20 minutes before baking, preheat your oven to 350°F (175°C). Prepare two baking sheets with parchment paper or silicone baking mats.

Form Cookies: Use a cookie scoop or tablespoon to scoop out chilled dough and roll into balls. Place them 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: Bake in the preheated oven for 10-12 minutes, or until the cookies are set around the edges but still soft in the center. You can optionally add a bit more fluff on top halfway through for extra gooeyness.

Cool: 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 and makes the cookies easier to handle.