Notes: Marshmallows can be very sticky. For the neatest bars, use a clean, sharp knife to cut the bars, rinsing and wiping the knife thoroughly between cuts.
- 1 cup graham cracker crumbs (made from 10 rectangular crackers)
- 12 tablespoons (1 1/2 sticks) unsalted butter
- 1/2 cup plus 1 tablespoon sugar
- 1 cup bittersweet chocolate chips
- 2 large eggs, at room temperature
- 1 teaspoon instant espresso powder
- 1 1/2 teaspoons vanilla extract
- 1/4 cup all-purpose flour
- 3/4 teaspoons baking powder
- 3 cups mini-marshmallows
Adjust oven rack to middle position and preheat oven to 350°F. 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.
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.
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.
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.
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.
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.