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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.
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Stir together the chocolate chips and 2 teaspoons of flour in a small bowl to coat the chips with flour.
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Whisk together the remaining flour, hot cocoa mix, cocoa powder, and salt in a bowl.
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Whisk together the sugar, butter, and eggs in a separate bowl.
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Add the wet ingredients to the dry ingredients and stir until just combined.
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Stir in the chocolate chips.
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Pour batter into the prepared pan and spread out the batter evenly.
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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.
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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.
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Cool brownies completely in the pan on a wire rack.
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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.
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Place the brownies on a cutting board and slice into squares.