Preheat to 350 degrees F. Grease an 8 inch metal pan or line with foil and spray with cooking spray.
Mix dry ingredients in a bowl; Set aside.Cream butter and sugars in a mixing bowl, using high speed of an electric mixer; beat in the egg and vanilla, beating for about 30 seconds or until thoroughly mixed. With a mixing spoon, stir in the flour in 3 parts, being careful not to over mix. When flour mixture is almost incorporated, add the walnuts, chips and marshmallows and continue stirring until all flour is absorbed.
Spread evenly in the prepared pan and bake for 25 minutes or until set.Makes 16 bars