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Preheat oven to 375 degrees.
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Line a 12-cup muffin pan with paper liners.
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In a large bowl, mix the flour, graham cracker crumbs, baking soda, and salt.
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In a medium bowl, whisk together brown sugar, egg, oil, and vanilla, until will blended. Whisk in buttermilk until blended.
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Add the egg mixture to the flour mixture and stir until just blended. Gently fold in chocolate chips and 1 cup of the marshmallows.
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Divide batter equally among prepared muffin cups. Sprinkle with the remaining marshmallow and gently press into patter.
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Bake in the preheated oven for 18 to 21 minutes, or until the tops are golden brown. Let cool in pan on a wire rack for 5 minutes, then transfer to a rack to cool.