Preheat oven to 350°F (180°C). Spray a 13x9-inch baking pan with cooking spray. Line pan with parchment paper, letting excess extend over sides of pan.

In a large bowl, whisk together brown sugar, melted butter, and vanilla for 30 seconds. Add eggs, one at a time, whisking until combined after each addition. Sprinkle baking powder onto mixture, and vigorously whisk until combined; repeat procedure with salt. Gently stir in graham cracker crumbs, flour, and 3/4 cup (127 grams) chocolate just until combined. (Do not overmix.) Spread batter into prepared pan.

Bake until top is browned and crackly, edges are set, and a wooden pick inserted in center comes out with a few moist crumbs, about 30 minutes. Increase oven temperature to broil. Sprinkle marshmallows and remaining chocolate onto blondie.

Broil on top rack until chocolate is melted and marshmallows are toasted, about 30 seconds, checking every 10 seconds. Using a wooden pick, swirl marshmallows and chocolate on top, if desired. Let cool completely in pan on a wire rack. Using excess parchment as handles, remove from pan, and cut into bars. Store in an airtight container for up to 3 days.