How to Make It
Combine marshmallows, butter, vanilla, and salt in a small saucepan over medium heat. Cook, stirring often, until melted and smooth. Remove from heat.
Lightly coat a large mixing bowl, a 9-inch baking dish, and a spatula with nonstick spray. (This helps prevent the treats from sticking.) Transfer marshmallow mixture to prepared bowl and add a few drops of food coloring, if using. Add cereal and fold until evenly coated in the marshmallow mixture. Transfer to prepared baking dish, pressing evenly into bottom of the dish. Let cool completely.
Turn treats out onto a work surface. Spray a shamrock-shaped cookie cutter with nonstick spray. Punch out shamrocks from treats or cut into squares. Shamrock Treats will keep, tightly wrapped at room temperature, for about 3 days.