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1
Position a rack in the lower third of the oven. Then preheat the oven to 325F/ 160C.
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2
Line the bottom and sides of an 8-inch square baking pan with parchment paper/ baking paper or foil, leaving an overhang on two opposite sides, to make it easy to lift brownies out of the pan.
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3
Place graham crackers or biscuits in a large Ziplock bag and seal bag. Lay bag flat on kitchen counter. Using a rolling pin, smash the biscuits until they are crushed to crumbs. Then roll until the crumbs are quite fine. (You may, instead, use a food processor for this step. I hate cleaning mine, so I use this method.) Add the brown sugar to the crumbs. Re-seal the bag and shake vigorously for several seconds to blend the sugar into the crumbs.
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4
Combine butter, sugar, cocoa, and salt in a medium heatproof bowl (like Pyrex) and set the bowl in a medium saucepan of barely simmering water. Stir almost constantly until the butter is melted and the mixture is hot but not burning. (It will look like wet dark brown sand. Don't worry, it will smooth out later.) Remove the bowl from the saucepan. Set aside briefly until the mixture is only warm, not hot.
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5
Stir in the vanilla with a wooden spoon. Add the eggs one at a time, stirring vigorously after each one. When the batter looks thick, shiny, and well blended, add the flour and stir until you cannot see it any longer, then beat vigorously for 40 strokes with the wooden spoon. Gently stir in 2 Tbsp of the graham cracker/ Granita biscuit crumbs. Very gently fold in 2/3 of the chocolate chunks.
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6
Spread brownie batter evenly in the lined pan. Smooth the top with a rubber spatula.
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7
Bake until a toothpick placed into the center comes out nearly clean, about 20 to 25 minutes. Allow brownies to cool almost completely on a rack, draped with a clean tea towel.
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8
Place marshmallows on top of the batter, leaving a small space around each one. They will spread and fill in the spaces, when warmed. Place the remaining 1/3 of chocolate chunks on top of the marshmallows. Meanwhile, place an oven rack about 6 inches below the top of the oven and preheat the broiler/ grill of the oven.
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9
Place brownie pan under broiler/ grill for 2-3 minutes, until marshmallows are golden brown. Make sure the paper liner does not get close to the heating element at the top of the oven, to prevent it from burning. Take brownies out of the oven promptly, when you see that the marshmallows have reached the desired golden hue. You must keep an eye on them. They will melt and spread and then turn brown quite quickly.
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10
Once brownies are out of the oven, sprinkle 1-2 Tbsp of the graham cracker/ Granita biscuit crumbs on top of the marshmallows.
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11
Allow brownies to cool completely on a rack, draped, again with the clean tea towel.
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12
Lifting up the ends of the parchment/ baking paper or foil liner, transfer the brownies to a large cutting board. Cut into 16 medium or 25 small squares.
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13
Store brownies in a tightly sealed container for up to 3 days. They probably won't last that long, though, since everyone will want to devour them immediately.
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14
If you like, serve brownies with extra graham cracker/ Granita biscuit crumbs sprinkled on top and on each plate. They would also be fabulous served with a scoop of vanilla ice cream.
