Preheat your oven to 180°C (350°F) for a conventional bake. For convection ovens, make any necessary adjustments (see note 1). Prepare three baking sheets by lining them with parchment paper. Two standard half-sheet pans and one smaller tray are ideal. Set aside.

In a medium-sized mixing bowl, sift together the all-purpose flour, cocoa powder, baking powder, baking soda, and salt. Whisk the dry ingredients thoroughly to ensure they are evenly distributed and set aside.

In a separate, large bowl, combine the light brown sugar, vegetable oil, egg, vanilla extract, and instant coffee powder. Whisk the mixture until it becomes smooth and well blended.

Gradually incorporate the dry ingredients into the wet mixture. Use a spatula to gently fold everything together until just combined. The batter will be thick, resembling cookie dough. Be sure to push the dry mixture into the wet ingredients until no streaks remain.

Pour in the hot milk and fold it into the batter until the mixture is uniform and smooth. The batter will transition to a thick cake-like consistency.

Scoop the batter onto the prepared baking sheets using a medium-sized spring-loaded cookie scoop (approximately 1½ tablespoons of batter per cookie). Space the portions about 2 inches apart. If the batter doesn’t form perfect rounds, don’t worry—you can use your fingers, lightly dipped in oil, to shape the edges for a neater appearance. This recipe should yield approximately 30 cookies.

Bake one tray at a time on the oven's middle rack for about 10 minutes. The cookies are done when their tops feel set and spring back when lightly pressed.

Allow the cookies to rest on the baking tray for 15 minutes before carefully peeling them off the parchment paper. Transfer them to a cooling rack and let them cool completely. Meanwhile, prepare the marshmallow buttercream.

In a large mixing bowl, combine the softened butter and powdered sugar. Using a hand mixer or a stand mixer set to medium-high speed, whip the mixture until it becomes light and fluffy.

Add the marshmallow fluff and vanilla extract, and continue mixing just until smooth. Take care not to over-mix, as this can affect the texture.

Turn half of the cookies upside down, revealing their flat sides. Using the same cookie scoop employed for the batter, add generous dollops of marshmallow buttercream onto the flat side of each cookie. Alternatively, you can pipe the frosting using a piping bag for a more polished look.

Place the remaining cookies on top of the frosted halves, flat side down, and gently press until the buttercream spreads evenly to the edges.