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

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

Add marshmallows, stirring frequently until melted.

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

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

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