Lay the jeans flat on a clean surface. Make sure that the butter stain is facing out and up.
Sprinkle an absorbent powder -- such as cornstarch, flour or baking soda -- over the greasy stain until it is lightly covered.
Wait 15 to 20 minutes to allow the powder to absorb the butter. Hold the jeans over a trash can and gently brush the powder off the fabric and in the trash can with a dry cloth.
Lay the jeans flat on a clean surface with the remaining butter stain facing out and up. Apply several drops of a prewash stain treatment directly to the jean material and rub with your fingertips for several seconds.
Wait 15 minutes before laundering and drying the jeans as you normally would.
Repeat Steps 4 and 5 until the popcorn butter is no longer present. Refrain from drying the jeans until you have removed the stains. The heat from the dryer will set the stain into the fabric.