PREHEAT oven to 350º
In a mixing bowl, cream butter and sugar until fluffy. Add vanilla and eggs, beating well after each addition.
Combine flour, cocoa, baking powder, salt and chopped nuts. Stir into egg mixture until well blended. Spread mixture in bottom of greased 9 x 13"(3.5L) pan.
Bake 15 to 20 minutes Remove from oven.
Sprinkle marshmallows evenly over layer in pan. Bake another 3 minutes. Remove from oven and let cool.
In small sauce pan, combine chocolate chips and peanut butter. Cook over low heat, stirring constantly, until melted.
Stir in cereal and spread mixture evenly on top of cooled marshmallow layer.
Refrigerate 2 hours or until firm. Cut into bars.