Melt the butter over low heat in a large pot. Stir in the marshmallows and cook until fully melted and no marshmallow white remains. Stir in the peanut butter until combined. Stir in the cereal until distributed evenly through the marshmallow mixture. Pour out onto a lightly greased rectangular baking dish (9 by 13 inches). Smooth out the top of the treats by moistening your hands with a little water and using the palms of your hand to press down gently. Don't press down too hard or else your treats will be too dense. Set aside to cool.
Once the treats have cooled, melt the chocolate over low heat. Cut the treats into even squares and quickly just shallowly dip the top side of the treats into the chocolate. Turn chocolate side up and set in a baking dish or on a rack until set.