Preheat your oven to 350°F (175°C).

In a bowl, combine flour, cocoa powder, baking soda, and salt. Set aside.

In a large mixing bowl, beat the softened butter, sugar, and brown sugar until creamy.

Mix in the egg and vanilla extract until well combined.

Gradually add the dry ingredients to the butter mixture. Stir until just mixed.

Gently fold in the mini marshmallows to the cookie dough.

Using a tablespoon, scoop out dough and form it into balls. Place them onto a baking sheet lined with parchment paper. Leave space between each ball for spreading.

Create an indentation in the center of each cookie and add a small dollop of chocolate fudge sauce.

Place the cookies in the preheated oven and bake for 10-12 minutes. The edges should be firm while the centers appear slightly soft.

Allow them to cool on the baking sheet for a few minutes before transferring to a wire rack to cool completely.