Gather 3 tablespoons butter, 1 package marshmallows, 6 cups crispy rice cereal, and a greased 9×13-inch pan
Melt the butter in a large pot over low heat
Add the marshmallows and stir until completely melted and smooth
Remove from heat
Stir in the crispy rice cereal until evenly coated
Press the mixture into the greased pan with a buttered spatula or wax paper
Let cool until set
Cut into squares and serve