Preheat oven to 350 degrees. Line an 8x8 baking pan with parchment paper or aluminum foil. If your using aluminum foil, spray with non-stick baking spray.
In a large bowl, mix all the dry ingredients; flour, graham cracker crumbs, salt, cornstarch, and baking powder; set aside.
In a stand along mixer fitted with a paddle attachment, cream together the butter and sugar. Add egg and vanilla, scraping down sides of bowl as necessary.
Next, add the bowl of dry ingredients and continue to mix.
If dough is a little too crumbly, add a splash of milk.
Press dough into prepared pan and bake for 25 minutes. Take pan out and sprinkle chocolate chips on top, then cover with marshmallow. Bake for an additional 5 minutes then turn on broiler to 525 degrees and broil for 2-3 minutes or until marshmallows turn golden brown.
Let bars completely cool before cutting into them. Pop them in the fridge to speed up the cooling process.