Preheat oven to 350°F and line a baking sheet with parchment paper.
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, chocolate chips, and mini marshmallows. (The dough will be dry, but it should hold its shape on the cookie sheet.)
Using a large cookie scoop (about ¼ cup), place the cookie dough on the lined baking sheet, slightly flattening each and leaving about 2 inches between each cookie.
Bake for 12-15 minutes, or until golden brown.
Let the cookies cool slightly before transferring to a wire rack to cool completely.