In a bowl, cream butter and sugar; beat in egg until light and fluffy. Combine flour, baking powder and salt. In another bowl, combine buttermilk and vanilla. Add dry ingredients to the creamed mixture alternately with buttermilk mixture; mix well. Fold in marshmallows and chips. Pour into a greased and floured 13x9-in. baking pan. Combine topping ingredients; sprinkle over batter. Bake at 350° for 30-35 minutes or until a toothpick inserted in the center comes out clean.