Preheat oven to 180 degrees C. Grease a 20x30cm baking dish.
In a small mixing bowl combine flour, cocoa, baking powder and salt.
In a large bowl cream butter and sugar; beat in eggs and vanilla. Stir flour mixture into the creamed egg mixture. Spread batter into the greased dish.
Bake for 15 to 20 minutes.
Sprinkle marshmallows over top of the baked bars; bake 3 minutes more. Cool.
Place chocolate chips and peanut butter in a microwave safe bowl. Melt chocolate chips and peanut butter in a microwave on high until melted together. Cook one minute at a time, stirring after each minute.
Stir rice bubbles into the chocolate mixture. Spread the chocolate mixture over the cooled bars. Chill, cut into bars and refrigerate.