Place marshmallows and butter into a microwave safe bowl.
Melt in microwave on high for 1 1/2 - 2 minutes. or put in a sauce pan and heat on stovetop.
Stir until marshmallows and butter are melted and smooth. If butter is not totally melted, microwave for an additional 15 seconds.
Pour in cereal.
Stir until cereal is totally coated with marshmallow mixture.
Pour into a 9 x 13 pan and press down into pan
Video
Notes
For me, the secret to a perfect Rice Krispie treats includes two things - more butter and more marshmallows. I use an entire bag of marshmallows and an entire cube (1/2 cup of butter). Don't skimp on this! This is more than the back of the Rice Krispie cereal box will tell you to use.
You can make marshmallow/butter mixture in the microwave in a bowl or in a saucepan on the stove top. Both work fine.