Preheat the oven to 350 degrees. Generously coat an 8-inch square baking pan with nonstick cooking spray.
In a large bowl, with an electric mixer, beat the sugar and butter together until smooth and creamy. 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.
Baked for about 30 minutes or until golden brown. Let bars cool slightly before cutting and enjoy!