For some reason, when I do my research on jams, I ended up using recipes from France. I used http://www.cuisineaz.com as my guidance. I once made with figues and used sugar instead of powdered sugar. The point is to let it cook in very low heat for about 1 hour.
1 package (10 plums)
3/4 cup or more of powdered sugar
1/2 of lemon juice
1. peel plums, cut in pieces, add all other ingredients and let it cook, covered for about 1 hour in low heat. Mix it every 10 minutes.
2. transfer in jar, once the jam is cool. Keep in refrigerator for 1 week.
Susana @ Delicioso y Divertido