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

In a mixing bowl with an electric mixer or a stand mixer on medium speed, beat butter and 2 cups of sugar until light and fluffy, about 2 minutes.

Add the eggs and vanilla extract, and mix to combine on medium speed. Reduce the mixer speed to the lowest setting and slowly add flour mixture. Mix until well-combined.

Place two pieces of plastic wrap on the counter. Add half the dough to the center of each one. Form the dough into a flattened disk shape and wrap with plastic wrap. Refrigerate until firm, about 1 hour.

While the dough chills, add remaining ½ cup of sugar to a bowl. Stir in the hot cocoa mix.

Rolling between your hands, form dough into 1¼-inch balls. Roll each ball in the hot chocolate sugar mixture and place on the lined baking sheet, about 3 inches apart. Use your finger to press an indentation into the cookie dough ball and stuff with three mini marshmallows.

Bake for 10-12 minutes until set, rotating halfway through. Transfer baked cookies to a wire rack to cool.

Store cookies between layers of parchment paper in an airtight container at room temperature for up to 1 week.