Heat oven to 350°F. Line two baking sheets with parchment.
Whisk flour, unsweetened cocoa, hot cocoa mix, baking soda, and salt in a bowl.
Cream butter, granulated sugar, and brown sugar until fluffy, 2–3 minutes.
Beat in eggs and vanilla. Mix until smooth.
Add dry ingredients to the bowl. Mix on low just until combined.
Splash in milk if dough seems dry; mix briefly to bring together.
Fold in chocolate chips and half the marshmallows.
Scoop 1.5-tablespoon mounds onto sheets, spacing 2 inches apart.
Bake 8–10 minutes until edges set but centers look soft.
Press a few marshmallows on top; return to oven 1–2 minutes to puff. Cool 5 minutes, then move to a rack.