S'mores Blondies

Preheat the oven to 350F. Line a 9x13-inch baking pan with parchment paper, leaving overhang on two sides.
In a large bowl, whisk the melted butter and brown sugar together until smooth.
Whisk in the eggs one at a time, then whisk in the vanilla extract.
Add the flour, baking powder, and salt, and stir with a spatula just until no dry streaks remain -- the batter will be thick, more like cookie dough than cake batter.
Fold in 3/4 cup of the crushed graham crackers, the chocolate chips, and 1 cup of the mini marshmallows (reserve the rest for topping).
Spread the batter evenly into the prepared pan, pressing it into the corners with a spatula.
Bake for 20 minutes, then remove from the oven and sprinkle the remaining 1 cup of mini marshmallows and remaining 1/4 cup crushed graham crackers evenly over the top.
Return to the oven and bake for another 6-8 minutes, until the marshmallows are puffed and golden brown on top and a toothpick inserted into the blondie (avoiding a marshmallow or chocolate pocket) comes out with a few moist crumbs.
Cool the blondies in the pan on a wire rack for at least 45 minutes -- they need this time to firm up enough to cut cleanly, since the marshmallow topping stays sticky while warm.
Lift out using the parchment overhang and cut into 16 squares with a knife lightly greased or run under hot water for clean cuts through the sticky marshmallow layer.

Grease or wet your knife before cutting -- the toasted marshmallow topping is very sticky and will pull apart with a dry blade.

Watch the marshmallows closely in the final minutes of baking; they go from golden to burnt quickly under the broiler-like heat near the top of the oven.