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Preheat oven to 375°F Lightly coat a cookie sheet with cooking spray. Mix flour and baking soda in a small bowl; set aside.
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With a spoon, mix butter, peanut butter, and brown sugar in a medium bowl until smooth. Add egg and vanilla, and beat until smooth. Add flour mixture, and stir until combined.
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Roll dough into 1-inch balls and flatten with a fork; bake 8 to 10 minutes. Remove from oven and let stand for 10 minutes, then place cookies in the refrigerator for 20 minutes.
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While cookies cool, place chocolate chips and butter in a microwave-safe bowl; heat about 1 minute, until just melted. Let stand for 30 minutes, or until slightly stiff.
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Heat marshmallow cream in microwave for 10 seconds to soften. Spread marshmallow on a cookie, chocolate on another and sandwich together. Store finished cookies in the fridge.