Heat oven to 350°F. Line an 8-inch square pan with parchment and lightly grease.
Melt the butter and let it cool for 5 minutes so it doesn’t scramble the eggs.
Whisk melted butter with granulated and brown sugars until glossy and combined.
Add eggs and vanilla; whisk until smooth. Stir in the milk.
In a separate bowl, whisk flour, hot cocoa mix, cocoa powder, baking powder, and salt.
Fold dry ingredients into the wet mixture just until no dry pockets remain.
Stir in chocolate chips and spread the batter evenly into the prepared pan.
Bake 23–28 minutes until the center is just set. Scatter mini marshmallows over the top for the last 5 minutes to puff and lightly toast.
Cool in the pan on a rack for at least 20 minutes. Lift out and slice into squares.