How to Make It
Preheat oven to 350° F. Line the bottom and sides of a 9-by-13-inch baking pan with aluminum foil; butter the foil. In a medium bowl, whisk together the flour, baking powder, and salt.
In a medium saucepan, melt the butter over low heat. Remove the pan from heat and stir in the brown sugar and vanilla. Whisk in the eggs until combined. Mix in the flour mixture until combined, then the pecans.
Spread the flour-nut mixture in the prepared pan and bake until golden and a toothpick inserted in the center comes out clean, 20 to 25 minutes. Sprinkle with the graham crackers, marshmallows, and chocolate chips and bake until the marshmallows are pale golden, 7 to 8 minutes more. Set the pan on a wire rack and let cool completely. Lift the blondies out of the pan, using the foil to help, and cut into 24 bars.
Storage suggestion: Keep the blondies at room temperature, between sheets of wax paper in an airtight container, for up to 5 days.