I'm surprised no one has asked me what's in the mason jar. When I first started living with my wife, she was all freaked out about it.
I thought it was cornstarch and water and now I am curious.
I was going to guess schmaltz. Have you ever heard of/used that? It's rendered chicken fat, and I've never dared to try it, but my New York Cookbook (by Molly O'Neill) champions it for all kinds of uses, including what you describe but also for "spreading on a slice of fresh rye bread with a sprinkling of salt."
I never knew there was a name for it, but yes, I do use chicken fat and also turkey, duck and goose fat too. When I buy either whole chickens or parts and I'm going to make chicken salad, I skin the chicken and trim as much of the fat off as I can. I then put the skins and trimmings on sizzle platters and broil them on the lowest level of my broiler. As the fat starts to render out of the skin, I drain it into jars and when the skins are golden brown on both sides, and there's no more fat left, I let the cooked skins cool and give my dogs a treat. They go absolutely crazy over it.
I've never had it on bread, but it's one of my secret ingredients in my chicken salad, and everyone who's tasted it, can't figure out why it tastes so good. When I tell them, they give me a strange look, but then accept it because it is so flavorful. Another use is when you want to make an amazing chicken gravy. Instead of making a roux with butter, you use the chicken fat instead, and it adds much richer flavor to the gravy. I do the same thing with my turkey gravy at Thanksgiving. It's become so popular with my family and friends, that I make a quadruple batch and give everyone a pint to take home with them.
But the king of fowl fats is goose and duck. One dish my grandmother use to make and was an absolute favorite of mine was
Duck Rillettes. It's made from slow simmered duck meat and duck fat and eaten cold on toast. I would take this to school and the other kids would look at me like I was completely insane. But it was soooooooooooooo good! I could eat duck almost every night and probably never get sick of it.