Preheat oven to 350°F and line a baking sheet with parchment paper.
In a bowl, cream together butter, sugar, and brown sugar until light and fluffy.
Add egg and vanilla extract; beat until smooth.
In a separate bowl, whisk flour, baking soda, salt, cocoa powder, and hot chocolate mix.
Gradually add dry ingredients to the wet mixture until fully combined.
Fold in chocolate chips and mini marshmallows.
Drop spoonfuls of dough onto the prepared baking sheet.
Bake for 10–12 minutes or until edges are set.
Cool on the sheet for 5 minutes, then transfer to a wire rack.