In a mixing bowl, cream butter and sugar. Add eggs and vanilla; beat until fluffy. Combine flour, baking powder, salt and cocoa; add to creamed mixture. Stir in nuts if desired. Spread in a greased jelly roll pan. Bake at 180ºC or 15-18 minutes. Sprinkle marshmallows evenly over cake; return to oven for 2-3 minutes. Using a knife dipped in water, spread the melted marshmallows evenly over cake. Cool. For topping, combine chocolate chips, butter and peanut butter in a small saucepan. Cook over low heat, stirring constantly, until melted and well blended. Remove from heat, stir in cereal. Spread over bars. Chill.