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

Combine Dry Ingredients: In a medium bowl, whisk together the flour, cocoa powder, baking soda, and salt. Set aside.

Cream Wet Ingredients: In a large bowl, use an electric mixer to cream the softened butter and brown sugar together until light and fluffy, about 2-3 minutes. Beat in the egg and vanilla extract until fully combined.

Combine Mixtures: With the mixer on low speed, alternately add the dry ingredients and the milk to the butter mixture, beginning and ending with the dry ingredients. Mix until just combined. Do not overmix.

Scoop Dough: Using a small cookie scoop (about 1.5 tablespoons), drop rounded portions of dough onto the prepared baking sheets, spacing them about 2 inches apart.

Bake: Bake for 10-12 minutes, or until the tops spring back when lightly touched. Allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.

Make Filling: While the cookies cool, prepare the filling. In a large bowl, beat the softened butter until smooth. Gradually add the confectioners’ sugar and beat until creamy. Beat in the marshmallow fluff, vanilla extract, and salt until the filling is smooth and fluffy.

Assemble: Once cookies are completely cool, spoon or pipe a generous amount of filling onto the flat side of half of the cookies. Top with the remaining cookies, flat side down, and gently press together to form a sandwich.