In a large saucepan over medium heat, melt 1/2 cup of butter while stirring continuously.
Add the entire 10-ounce bag of mini marshmallows and stir until completely melted and smooth, about 2-3 minutes.
Remove from heat and fold in 5 cups of Rice Krispies cereal until evenly coated. Add optional mini marshmallows if desired.
Transfer the mixture to a greased 9x13 inch baking pan and press down firmly to create an even layer.
Allow to cool at room temperature for about 30 minutes, then cut into squares or rectangles.
