Grease a 9 x 9 or 9 x 13 inch pan with cooking spray or butter. Set aside.
In a large pot or Dutch oven, melt the butter over medium heat. Stir in the sweetened condensed milk and cook until bubbly, about 1 minute.
Lower the heat to low and add the splash of vanilla and salt. Add the 10 ounces of marshmallows and heat, stirring frequently, until the mixture is melted and smooth.
Pour in the Rice Krispie treats, stirring gently until the marshmallow mixture and the cereal start to combine. Add the remaining cup of marshmallows (if using) and stir just until distributed and starting to melt.
Transfer the mixture to the prepared pan. Spray your hands with cooking spray, then gently spread the rice krispie treat evenly over the pan. DO NOT PRESS DOWN!
Meanwhile, melt the chocolate with the coconut oil in the microwave, stirring every 30 seconds. Once the chocolate is melted, spread it evenly over the prepeared treats. Allow it to set up for a few hours before slicing.