Prepare 9×13 or 9×9 pan by spraying lightly with cooking spray. (The size of the pan depends on the thickness you prefer your krispie treats.)
In a large saucepan over low heat melt butter.
Once butter is melted dump in your mini marshmallows.
Stir until marshmallows are completely melted.
Once melted, remove from heat and stir in your krispie cereal.
Immediately press into your prepared pan.
Let cool about 15 minutes and cut into squares.
I prefer thicker krispies and either double the recipe and use a 9×13, or use a 9×9 for the recipe “as-is”. A tip to pressing the krispies in the pan evenly is spray a piece of wax paper lightly with cooking spray and use the sprayed size to evenly press in prepared pan.