Heat oven to 350°F. Stir Graham craker crumbs, 1/4 cup cocoa and sugar in medium bowl; stir in butter, blending well. Press mixture firmly onto bottom of ungreased 13x9x2-inch baking pan.

Beat sweetened condensed milk, flour, egg, vanilla, baking powder and remaining 1/4 cup cocoa in small bowl. Stir in 1/2 cup nuts. Spread evenly over prepared crust. Sprinkle with chocolate chips.

Bake 22 minutes. Sprinkle the other 1/2 cup of nuts, a handful or so more chocolate chips, and the mini marshmallows. Continue baking 5 minutes or until marshmallows have softened and puffed slightly.

Cut into bars. Store tightly covered at room temperature.