Cream butter and sugar; beat in eggs and vanilla. Stir together flour, chopped nuts, cocoa, baking powder and 1/4 teaspoon salt. Stir into egg mixture. Spread in bottom of greased 13 x 9 x 2-inch baking pan. Bake in 350 degrees oven for 15 to 20 minutes or until bars test done. Sprinkle marshmallows evenly atop. Bake 3 minutes more. Cool. In small pan combine chips and peanut butter; cook and stir over low heat until chips are melted. Stir in cereal. Spread mixture over cooled bars. Chill and cut into bars. Refrigerate. Makes 3 to 4 dozen bars.