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1
Preheat oven to 350 F.
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2
To make the crust: Combine the graham cracker crumbs, sugar, and salt in a medium bowl - stir with a fork to combine. Drizzle the melted butter over the top, then mix until all of the crumbs are evenly moistened. Transfer to the prepared pan and use your fingertips to press the crumbs into an even layer over the bottom of the pan. Bake for 8-10 minutes, or until firm and golden brown. Remove the pan to a wire rack and let the crust cool as you prepare the brownie batter.
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3
To make the brownies: Add the butter and chocolate to a heatproof bowl. Microwave in 30 second bursts, stirring in between each, until both the chocolate and butter are completely melted, and the mixture is smooth. Set aside to cool slightly. In a medium bowl, whisk the flour, baking powder and salt together. In a second medium bowl, whisk the sugar, eggs and vanilla extract until combined. Add the cooled chocolate mixture to the sugar/eggs, and whisk to combine. Use a rubber spatula to fold the flour mixture into the batter, mixing only until the flour is just incorporated.
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4
Transfer the batter to the baking pan and gently spread into an even layer on top of the graham cracker crust. Drop small dollops of the peanut butter over the batter. Carefully use the tip of a knife to swirl the peanut butter and brownie batter together. Bake for 22-27 minutes, or until a toothpick inserted in the center comes out with just a few moist crumbs attached. Remove the pan to a wire rack and set the oven to broil.
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5
Top the brownies with the mini marshmallows. Return them to the oven and broil just until the marshmallows have puffed and browned slightly, about 1-2 minutes. Transfer the pan to a wire rack and let the brownies cool completely, at least 2 hours.
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6
Use the foil handles to lift the brownies out of the pan and transfer to a cutting board. Cut into squares for serving - you can spray your knife with nonstick cooking spray to keep the marshmallows from sticking to it.
