In a pot over low heat, melt butter and marshmallow together until smooth. Stir in peanut butter and stir until combined. Pour over the cereal, add bacon, and toss to make everything is coated. Let cool slightly - enough so the chocolate won't melt but you still want it to be soft enough to work with. Quickly fold in most of the chocolate. Transfer to a 8x8-inch pan and press remaning chocolate chunks into the top. Cool completely before serving.