Melt butter in a large stock pot. This should be done slowly over low to medium heat.
Add marshmallow fluff to melted butter and stir them together. Keep this on low to medium heat while it all melts together.
Alternatively, you can combine both the butter and the fluff into a microwave safe bowl. Then pop them in the microwave for 30 - 60 second increments. Stirring in between each heating. Until the whole mixture is melted together.
add in Rice Krispies and mix thoroughly
Thoroughly and evenly mix the marshmallows and Rice Krispies together.
Spread the mixture into a greased pan with a greased spatula.
Allow mixture to cool (about 30 minutes), cut into squares and enjoy!