In a large saucepan, melt the butter over low heat.
Add the marshmallows to the melted butter and stir continuously until they are completely melted and smooth.
Remove the saucepan from the heat.
Pour the Rice Krispies cereal into the saucepan with the melted marshmallow mixture.
Using a spatula or wooden spoon, gently fold the cereal into the marshmallow mixture until all of the cereal is evenly coated.
Transfer the mixture into a greased 9x13-inch pan or a pan lined with parchment paper.
Use the spatula or a piece of wax paper to press the mixture firmly and evenly into the pan.
Let the treats cool at room temperature for about 30 minutes, or until they are set.
Once cooled and set, cut the treats into squares or your desired shapes.
Enjoy your homemade Rice Krispie treats.