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1. Adjust oven rack to middle position and preheat oven to 350°F (175°C).
2. Cut two 16-inch (41cm) pieces of foil and line an 8-inch (20cm) square baking pan, allowing excess to hang over sides.
3. Spray with nonstick cooking spray.
4. Break graham crackers into small pieces and pulse in food processor until they become fine crumbs.
5. Add melted butter, sugar, and 1/4 teaspoon salt to crumbs and pulse to combine.
6. Press into bottom of prepared pan.
7. Melt chocolate and butter in medium bowl in microwave, 1 to 2 minutes, stopping every 30 seconds to stir with rubber spatula.
8. Let cool.
9. With a wooden spoon, mix in the remaining sugar, eggs, espresso powder, vanilla, and 1/4 teaspoon salt.
10. Add the flour and baking powder and mix until smooth.
11. Pour batter into prepared pan.
12. Bake for 25 to 30 minutes, until a toothpick inserted into the center of the brownies comes out with moist crumbs attached.
13. Do not over-bake.
14. Transfer to cooling rack and top with marshmallows.
15. Adjust oven rack to upper third position and heat broiler.
16. Broil until marshmallows are golden brown, 1 to 3 minutes.
17. Let the brownies cool until they're just warm enough to handle.
18. Remove foil and cut into squares.
19. Store in an air-tight container at room temperature for up to 3 days.

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