Preheat oven to 350°F and spray a 9x13-inch pan with nonstick cooking spray.
In the bowl of a stand mixer, add butter, granulated sugar, and brown sugar. Mix on medium until creamy (about 2 minutes).
Add egg and vanilla and continue mixing for 8 minutes. (Yes, 8 minutes, to aerate the dough.)
Add all-purpose flour, bread flour, baking soda, baking powder, and salt and mix on low until just incorporated.
Gently fold in the cooled cornflake crunch.
Divide dough in half and place half into the base of the 9x13-inch pan. Press down into an even layer.
Top with Hershey bars and mini marshmallows.
Top with the other half of dough in an even layer (all the way to the edges).
Bake for 25-30 minutes, or until golden brown.
Let the bars cool before serving.