Blot the spill immediately with paper towels, using a fresh piece of paper towel each time you blot to prevent the butter from spreading to other areas of the shirt.
Squirt a few drops of shampoo onto a clean paper towel and continue to blot over the shirt to remove any leftover butter. Continue until all butter is removed, then run the shirt under water to remove the shampoo residue.
Apply a stain lifter to the shirt if any butter residue remains on the shirt. Let the stain lifter sit for about 30 minutes, then launder the shirt as you normally would with your regular laundry detergent.