Spray an 8x8-inch baking dish with nonstick cooking spray. Set aside.

In a large pot over medium heat, melt butter. Add sweetened condensed milk and stir to combine. Bring the mixture to a simmer, stirring constantly to prevent burning.

Add marshmallows, stirring frequently until melted.

Remove the pot from heat and fold in the rice cereal with a clean rubber spatula. Stir until well coated.

Transfer mixture into the prepared baking dish and press into an even layer.

Allow the bars to cool to room temperature before cutting and serving.