Wash the nannyberries well and drain, then combine with the water, bring to a simmer, and cook, covered, for 40-45 minutes on low heat, mashing them up to make a thick puree.
Depending on how fresh or dry your fruit are, you may need to add more water to get the fruit really mashed up. It’s ok if the mixture seems a little soupy or loose, since you can always cook the puree down slowly afterward to concentrate it. Dried nannyberries may need more water than very fresh ones to become a puree, so use your instinct.
Pass the mixture through a foodmill with a large die (large holes). My largest food mill screen is just barely small enough to not allow the seeds through, but some aren’t.
If the mixture is liquid enough, I’ve had success pulsing it with a handblender or in a food processor to get extra puree from the nannyberries, but you need to be careful of the seeds as they’re brittle. If you pass your nanny butter through a food mill and still see a seed or two, pass it through a finer screen before storing.
Nanny Butter
For nannyberry butter add 1 cup maple to the 5 cups of puree along with ¼ teaspoon cinnamon and a pinch of salt, then puree in a blender in batches to make it extra smooth, pour it into a shallow pan and bake, whisking occasionally for about 30 minutes at 325 or until thickened.
Double check the seasoning, adjust until it tastes good to you, then store in a jar in the fridge for up to a week, or freeze.
Notes
Nannyberry butter is wonderful made with only nannyberries. But, for a change of pace, or if you have only a small amount of fruit you can mix nannyberries with apples and make a similar butter.