Gather a large saucepan, a sturdy spatula, and a 9x13 inch pan lined with parchment paper or lightly greased.
Melt the butter in the saucepan over medium heat, stirring occasionally. Let it cook for about 2-3 minutes until it smells rich and nutty, and the butter is fully melted.
Add the mini marshmallows to the melted butter. Stir constantly with the spatula until the marshmallows are completely melted, glossy, and smooth. You'll hear a gentle bubbling and see the mixture turn shiny.
Remove the saucepan from heat and stir in the vanilla extract and salt. Mix until well combined and fragrant.
Pour in the rice cereal gradually, folding gently with the spatula until the cereal is evenly coated with the marshmallow mixture. The mixture will be thick and sticky, pulling away from the sides of the pan.
If you're using chocolate chips or colorful add-ins, fold them in now for a fun, chaotic look.
Transfer the mixture into the prepared pan. Using the spatula or your hands (lightly greased if needed), press the mixture firmly into an even layer. The mixture should be slightly sticky but hold together well.
Let the treats sit at room temperature for about 30 minutes to set and firm up. The edges may start to turn a slight golden brown, and the surface will look glossy and chewy.
Once set, cut into squares or your preferred shapes. The treats will be chewy, gooey, and delightfully chaotic-looking with colorful bits poking out.
Enjoy your wild, playful monster rice krispie treats as a fun snack or dessert, perfect for sharing and customizing with your favorite add-ins!