Preheat the oven to 350 degrees. Line an 8-inch square baking pan with parchment paper, with 2 sides having a slight overhang for easy removal.
In a large bowl, with an electric mixer, beat the butter and sugar together until smooth and creamy, about 2 minutes. Add in the egg and vanilla until incorporated. Add in the graham crackers, flour, salt, and baking powered until combined. (Dough will be sticky.)
Coat the back of a spatula with nonstick spray; spread half of the dough into the pan evenly. Sprinkle the Kit-Kat pieces over the dough, followed by the marshmallows. Flatten pieces of the remaining dough and place them on top of the marshmallows (leaving some of the marshmallows exposed, so as it bakes, it has a cobbled look.)
Bake for about 25 minutes or until golden brown. Let bars cool completely before cutting and enjoy!