Heat 2 cups chocolate chips and butter in a saucepan over low heat, stirring constantly, until melted, remove from heat. Stir in sugar, flour, vanilla, baking powder, salt and beaten eggs.
Spread batter in an un-greased 13x9 inch baking pan. Bake 27-32 minutes or until center is set.
Immediately top with marshmallows, 1 cup chocolate chips and nuts, pressing into brownies slightly. Set oven temperature to broil. Broil brownies 4 inches from heat 1 to 2 minutes or until marshmallows are puffed and light golden brown. Cool completely in pan on a wire rack.
Cook's Note: Place bars in the refrigerator after cooling completely. This will make it easier to cut the bars. I like to use a plastic knife to cut the brownies into bars.