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 pan. Bake for 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, 3 Tb butter and peanut butter in a small saucepan. Cook over low heat, stirring constantly, until melted and well blended. Remove from heat; stir in rice bubbles. Spread over bars. Chill.