Preheat oven to 350 degrees F. Grease an 8-inch square baking pan. Line the pan with parchment paper with about a 1-inch overhang. Grease the parchment paper.
Stir together the chocolate chips and 2 teaspoons of flour in a small bowl to coat the chips with flour.
Whisk together the remaining flour, hot cocoa mix, cocoa powder, and salt in a bowl.
Whisk together the sugar, butter, and eggs in a separate bowl.
Add the wet ingredients to the dry ingredients and stir until just combined.
Stir in the chocolate chips.
Pour batter into the prepared pan and spread out the batter evenly.
Bake for 30 to 35 minutes, or until a toothpick inserted in the center comes out with a little batter and some moist clumps clinging to it. Do not over-bake.
Scatter the mini marshmallows over the warm brownies. Wait 1 minute for the marshmallows to soften a little and use your fingers to gently press down on the marshmallows.
Cool brownies completely in the pan on a wire rack.
Run a knife between the brownies and the pan on the sides without parchment. Lift the brownies out of the pan using the parchment paper.
Place the brownies on a cutting board and slice into squares.
