Spray a 9×9-inch pan with non-stick spray and line it with parchment paper. Spray again.
In a very large microwave-safe bowl, or in a pot on the stove over medium heat, place your butter, salt, and 8 cups/400 grams of mini marshmallows. Microwave in 30-second increments, or heat, stirring in between, until everything is completely melted and smooth.
Add vanilla and stir again.
Add the rice krispies and mix well.
Add in the remaining marshmallows, and Eggies if using, while still warm and lightly press into your prepared pan. Have a small bowl of water handy and dip your hands into it before quickly and firmly pressing the mixture into the pan. Dip your hands into the water frequently to prevent sticking!