In a small bowl, cream butter and sugar until light and fluffy. Add eggs, one at a time, beating well after each addition. Beat in vanilla.
Combine the flour, cocoa, baking powder and salt; gradually add to creamed mixture. Stir in nuts if desired. Spread in a greased 15-by-10-by-1-inch baking pan.
Bake at 350 degrees for 15-18 minutes.
Sprinkle with marshmallows; bake 2-3 minutes longer. Remove to a wire rack. Using a knife dipped in water, spread the melted marshmallows evenly over top. Cool.
For topping, combine the chocolate chips, peanut butter and butter in a small saucepan. Cook and stir over low heat until blended. Remove from the heat; stir in cereal. Spread over bars immediately. Chill.