1. Begin by rinsing off the plums. Slice each in half, and remove the pit. Set aside.

2. Measure out the plums to that you have 10 cups of halved plums total. Note: To measure the plums: pack the halved plums into a large measuring cup.

3. In a large wide and shallow pot, combine plums and sugar. Bring the mixture to a boil, stirring frequently. Add the butter.

4. Reduce the heat, and cook the jam for 20-30 minutes, stirring to prevent any burning. Cook until the temperature reaches 220 F, or sets using the cold spoon trick.

5. While the jam cooks, sterilize and prepare your jars & water bath: rinse lids and rims in warm soapy water. Dry and set aside on a clean surface. Fill a large pot with warm water.

6. Once the jam is ready, ladle hot jam into sterilized jars. Add lids and tighten rings “finger tight” so that any air bubbles can escape.

7. Place each jar in the large pot filled with hot water. Bring the water to a boil. Process in the boiling water bath for 15 minutes (adjusting for altitude if necessary). Make sure the water covers the jars by 1-2” in the water bath.

8. Once the jam has been processed, set on a towel or cutting board and leave for 12-24 hours. You should hear popping sounds signaling a good seal. After 24 hours, check the lids to make sure they’ve sealed.