Preheat Oven
Preheat oven to 350°F (175°C). Line baking sheets with parchment paper.
Mix Dry Ingredients
In a bowl, whisk flour, cocoa powder, hot chocolate mix, baking soda, and salt.
Cream Butter & Sugars
Beat butter with brown sugar and granulated sugar until light and fluffy.
Add Egg & Vanilla
Mix in egg and vanilla extract until combined.
Combine
Gradually add dry ingredients to wet ingredients, mixing just until incorporated.
Fold in Add-Ins
Gently fold in chocolate chips and mini marshmallows.
Bake
Scoop dough onto baking sheets and bake for 9–11 minutes, until edges are set and centers are soft.
Cool Slightly
Let cookies rest on the pan for 5 minutes before transferring to a rack.