Never have heard of them, so of course I had to go look!
Hackberries: Nature’s Grape-Nuts for Winter Survival
https://thegrownetwork.com/hackberries-winter-survival/
So how do you eat them? You’ve got a few options. You can eat the entire thing raw. Just crush it up with your teeth. (Or crush the seeds up with a mortar and pestle, or in a blender with a little water. That would be smartest and easiest.) You could also just scrape off the sweet pulp and spit out the seed, though you’ll be eating a whole lot of hackberries before you get full that way.
The seeds can be crushed and soaked in water (skin on or off) to make a nut milk, similarly to how you make homemade almond milk. Put the crushed seeds in a cup and pour in enough water to cover them. Let it sit overnight. Then strain the liquid through a cloth to catch the bits of seed. Be sure to squeeze the pulp to get out all of the liquid. When I’ve tried this, the results were good, but not as flavorful as almond milk. I expect that the results would improve as you refine your technique.
My favorite hackberry concoction is “Fake-Nuts,” or homemade Grape-Nuts cereal using hackberries. I usually eat the skin and pulp first and just wash the seeds off to get rid of any mouth germs before serving them to my kids. If that sounds gross, just leave the skins on. It will add extra sweetness.
Put the seeds in a blender with a little water and pulse them until you’re reasonably sure that you’ve broken them all up. Then pour them into a skillet with a little oil, and simmer them on the stovetop until the water boils away and you’ve got an oatmeal-like consistency. The heat helps to weaken the shells, making them much easier to chew, similar to Grape-Nuts cereal. But be aware that you occasionally get a stubborn piece, so don’t go crazy.
This concoction is fair on its own, though rather bland unless you’ve left the skins on. I like to add a little maple syrup and perhaps a splash of milk or cream to complement the texture. It makes a good, warming breakfast on a cold winter morning.