Add mini marshmallows and salted butter to a microwave-safe bowl.
Heat for 1 minute to start.
Then stir until marshmallows and butter are mostly incorporated. If you still see liquid butter, heat for another 30 seconds in the microwave.
Stir until mixture is smooth. Next add the cereal. Use a big spoon or spatula and stir until combined.
Add Rice Krispies to a 9" x 13" pan prepared with parchment paper and nonstick spray. Spread it out in an even layer, pressing the Rice Krispies into the pan with your hand or a spatula.
Wait one hour until the dessert has hardened, then serve. Cut each square as you serve it, so the treats don't dry out.
Video
Notes
In the microwave, you'll want to heat the mixture until it is just melted. Add 30 seconds of heating time, stirring thoroughly each time, until melted.
Use both parchment paper and nonstick spray in the treats pan. The mixture is super sticky, so this will help once it is cooled and ready to serve.
Coat your hands or a spatula in nonstick spray to make it easier to press the super sticky mixture into the pan.