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1
Adjust oven rack to middle position and preheat oven to 350u00b0F. Cut two 16-inch pieces of foil and line an 8-inch square baking pan, allowing excess to hang over sides. Spray with nonstick cooking spray.
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2
Break graham crackers into small pieces and pulse in food processor until they become fine crumbs. Melt 4 tablespoons butter. Add melted butter, 1 tablespoon sugar, and 1/4 teaspoon salt to crumbs and pulse to combine. Press into bottom of prepared pan.
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3
Melt chocolate and remaining 8 tablespoons butter in medium bowl in microwave, 1 to 2 minutes, stopping every 20 seconds to stir with rubber spatula. Transfer mixture to a large bowl and cool, stirring occasionally, for 10 minutes.
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4
With a wooden spoon, mix in the remaining 1/2 cup sugar, eggs, espresso powder, vanilla, and 1/4 teaspoon salt. Add the flour and baking powder and mix until smooth.
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5
Pour batter into prepared pan. Bake for 25 to 30 minutes, until a toothpick inserted into the center of the brownies comes out with moist crumbs attached. Do not over-bake. Transfer to cooling rack and top with marshmallows.
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6
Adjust oven rack to upper third position and heat broiler. Broil until marshmallows are golden brown, 1 to 3 minutes. Cool in pan 1 hour. Using foil sling, transfer directly to wire rack and let cool completely, at least 1 hour. Cut into 2-inch squares and serve.
