S'mores Brownies

Adjust oven rack to middle position and preheat oven to 350°F (180°C). Line an 8- by 8-inch baking pan with two 8-inch-wide strips of foil so that all sides are covered with about 3 inches of overhang on all sides. Using nonstick cooking spray, grease foil.

Break graham crackers into small pieces and, using a food processor, pulse until finely ground. (You should have about 1 cup of crumbs.) In a small microwave-safe bowl, microwave 4 tablespoons butter on medium power, stirring occasionally, until melted, about 1 minute. Add melted butter, 1 tablespoon (15 g) sugar, and 1/4 teaspoon salt to crumbs and pulse to combine. Using a flexible spatula, scrape crumbs onto bottom of prepared pan. Spread into an even layer, then use a flat-bottomed drinking glass or measuring cup to compress firmly until crumbs are in a compact, even layer across bottom and sides of pan.

In a large microwave-safe bowl, combine chocolate and remaining 8 tablespoons (4 ounces; 113 g) butter; microwave on high, stirring every 30 seconds, until melted and smooth, about 2 minutes.

Whisk in remaining 1/2 cup (100 g; 3 1/2 ounces) sugar, eggs, espresso powder, vanilla, and remaining 1/4 teaspoon salt. Add flour and baking powder, and whisk until smooth.

Pour batter into prepared pan. Bake until a toothpick or cake tester inserted into the center of the brownies comes out with moist crumbs attached, 25 to 30 minutes. Transfer to a wire rack and top with marshmallows.

Using gloved hands or kitchen towels, carefully adjust oven rack to upper third position and heat broiler. Return brownies to oven and broil until marshmallows are golden brown, about 1 minute.

Transfer to a wire rack and let cool in pan until room temperature, about 1 hour.

Using a chef's knife lightly greased with cooking spray, slice and serve brownies.