In a pot over low heat, add in 6 cups marshmallows and butter, stir frequently until it's smooth.
In a large bowl, add in the graham cereal and stir in the warm marshmallow mixture over them. Stir in the remaining marshmallows.
To a greased 9 x 13 baking dish, pour in half of the mix from step 3 and press into the pan. Sprinkle half of the chocolate pieces. Add the remaining graham marshmallow mix and top with the rest of the chocolate.