Preheat the oven to 350°F. Line two baking sheets with parchment paper and set aside.
Cream the butter and sugar in a large bowl until light and fluffy.
Add the hot cocoa mix, instant coffee (if using), eggs, and vanilla. Stir until smooth and well combined.
Mix in the flour and baking soda just until no dry pockets remain. Do not overmix.
Fold in the chocolate chips.
Scoop the dough onto prepared baking sheets, using about 2 tablespoons of dough per cookie. Space cookies well apart (about 6 per sheet).
Bake for 10–11 minutes, until the edges are set but the centers are still soft.
Add marshmallows. Immediately after removing from the oven, gently press the back of a spoon into the center of each cookie to create a small indentation. Press a few mini marshmallows into each cookie.
Cool. Let cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.
Decorate. Once cooled, drizzle with melted chocolate and top with sprinkles if desired.