Cream butter (margarine) and sugar. Beat in the eggs and vanilla. Stir together flour, chopped nuts, cocoa powder and baking powder and stir into egg mixture. Spread in bottom of greased 13 by 9 by 2-inch pan. Bake in 350 F. oven for 15 to 20 minutes. Sprinkle marshmallows evenly on top; bake 3 minutes more and cool. In small saucepan com bine chocolate chips and peanut butter. Cook and stir over low heat until chocolate is melted. Stir in cereal. Spread mixture over cooled bars. Chill. Cut into bars. Refrigerate.