1. 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 aside.
2. Cream Butter and Sugar: In a large mixing bowl, use an electric mixer to cream 1 cup room temperature unsalted butter with 1½ cups granulated sugar until light and fluffy, about 3-4 minutes.
3. 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 vanilla extract.
4. 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. You can lightly swirl the marshmallow fluff (or melted marshmallows) at this point or wait until after chilling the dough.
5. Chill the Dough: Cover the dough and refrigerate for at least 1 hour to make it easier to handle and to prevent excessive spreading during baking.
6. Preheat Oven: About 20 minutes before you’re ready to bake, start preheating your oven to 350°F (175°C). Prepare two baking sheets with parchment paper or silicone baking mats.
7. 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 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. Then, use a toothpick to swirl the marshmallow into the cookie.
8. 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. Optionally, add a bit more fluff on top halfway through for extra gooeyness.
9. Cool on Pan: Allow 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.