Melt the butter over medium heat. Once the butter is melted the browning will happen quickly. The top will be covered in white foam, that typically means it has browned. Use a wooden spoon to move the foam aside and check the color. It goes from brown to burnt quickly so don’t walk away and be ready with the marshmallows.
Once the butter is browned immediately add the marshmallows. Reduce the heat by half. Stir until completely melted.
Remove from the heat and add the rice cereal. Stir until completely combined.
Pour the rice cereal mixture into a prepared 9x13 baking pan. Smooth top with rubber spatula.