Add the chopped plums, sugar, vinegar, soy, garlic, ginger, and sriracha to a pot over medium low heat. Cook for 20-30 minutes, stirring occasionally. Allow to cool, then blend on high for 3 minutes or until smooth.
Preheat oven as high as it will go, at least 500°F.
Sprinkle the ducks with kosher salt, then brush them with the sauce. Put the ducks on top of a rack in a baking tray, or onto some celery stalks in a cast iron skillet.
Cook for 15 minutes. Remove the tray and brush more sauce onto the ducks, then cook for another 10-20 minutes, or until a meat thermometer stuck deep into the breast reads 140°F.
Let rest for 10 minutes before carving and serving.
