Preheat oven to 350°F (175°C). Line two baking sheets with parchment paper.

In a medium bowl, whisk together the flour, cocoa powder, baking soda, and salt.

In a large bowl, cream the softened butter, brown sugar, and granulated sugar together until light and fluffy (about 2-3 minutes).

Add the egg and vanilla extract to the butter mixture, beating until well combined.

Gradually mix in the dry ingredients on low speed just until no streaks of flour remain. Do not overmix.

Scoop dough into 1.5 tablespoon-sized balls, place on baking sheets 2 inches apart, and flatten slightly into thick discs.

Bake for 9-11 minutes until the edges are set but the centers are still very soft. Let cool completely on a wire rack.

Match cookies into pairs. Spread marshmallow fluff on the bottom of one cookie, and Nutella on the bottom of its pair.

Gently press the two halves together to create a sandwich. Serve immediately or store in an airtight container.