Line a 9x13 baking dish with parchment. Line the parchment with a thin coating of non-stick spray and set aside. This will make it easier to spread the rice cereal.
In a large pot, melt the butter over medium heat. Once the butter is fully melted, add in the marshmallows. Add in the vanilla and the salt. Stir until the mixture is fully melted. It will look thick, but a little soupy. If you want to add some candy to the middle of the treats, do it now. The candy will most likely melt, but add more flavor depending on what candy you put in.
Pour the mixture is in the prepared baking dish. Flatten it out with a spatula or use a separate piece of parchment paper to flatten it out. Top the treats with chopped up candy pieces and press them down into the treats. Add as many as you would like.
Let the treats sit out for about an hour before cutting into them. If the chocolate gets very melty during the process, you can place the treats in the refrigerator to set.
Using the sides of the parchment, lift the treats out of the pan. Cut into squares using a warm knife. The treats will last about 3-4 days in an airtight container.
