Cut your plums in half, then in quarters, then into eighths. Keep the skins on; just dispose of the pits.
In a medium-size pot, bring the plums and 2 tablespoons of powdered sugar up to a boil, let it boil for 2-3 minutes, and then turn off the heat and let it cool down to room temperature, which should take about 15 minutes.
Check on the sweetness level by dipping a spoon into your jam and then taste-testing the jam. If your jams were less ripe to begin with, you may want to add another teaspoon of powdered sugar. If your plums were riper to begin with, then the jam might be sweet enough as is. I often add 1 teaspoon of sugar at this point.
Bringing the plums to a boil a second time. If your plums have not broken down yet, using your spatula, just smush/divide the plums into smaller broken down pieces. Really keep on an eye on the jam, and stir it almost constantly. Do not set it and forget it. No Instagram surfing. Watch the pot. Boil for 2-3 minutes, then turn off the heat and let it cool down for another 15 minutes.
At this point, the plum jam should be nearly ready. If you used less-ripe plums, which contain more pectin, it should be ready after just these 2 boils. Do a spoon test -- dip a spoon into the jam, and drag your finger down the middle. If the jam stays separated, it's ready!
If it's still too loose, bring it to a boil once more. Again, watch the pot!! Do not burn the jam!!
Let the jam cool down completely and pour the jam into a 16 oz jar, and store in the fridge.
If you know how to do a boiling-canning process, do that and the jam will last up to a year. If not, the jam will last about 2 weeks in the fridge. Serve on toast, with yogurt, on ice cream, in between cake layers, etc!!