Liver is VERY popular with dogs. Most dogs I know will do anything for some liver.
That said, you don't have to get fancy. My dogs ultimate treat is cheese. She will do literally anything for a bite of cheese. So I buy cheese sticks and feed her tiny little bites when we're training for something that needs a jackpot reward. Some people use shredded chicken or hot dog pieces or even little bacon pieces. All of those are better quality than Pupperoni. (although if its only occasional, it wont harm your dog either).
I'm just saying, you don't need to bake special dog treats, when they probably prefer the raw ingredients anyway
For commercial treats, Cloud Star makes some soft training treats, and the soft ginger bread men break into tiny pieces easily. Also Zuke's Training treats are a big hit, although the salmon ones are stinky (which makes them even tastier to dogs, lol) Nutro treats are pretty good. Nature's Recipe also makes some soft venison treats that are nice (but I don't like other treats in their line). And you can buy all sorts of brands of various freeze dried livers (chicken, beef, lamb) which crumbles easily and is beloved by dogs, if you want a to-go version of your liver treats. My dogs absolute favorite commercial treat is Evanger's Nothing But Naturals. They're like doggy crack. The Evangers rep gave out free bags to our customers at my last job before we got them in stock, when they first came out, and those customers came back in droves demanding to know when we were getting them in because their dogs were CRAZY about them. All of those brands are all natural.
I follow most of the same rules for dog treats that I do for dog food. I avoid treats with BHA, BHT, or Ethoxyquin. Also no treats with by-products or unnamed meats (like poultry, instead of saying chicken, turkey etc) Mixed tocopjerols are very good, its a natural way to perserve food. I'm a bit more lax about wheat and corn in my dog treats, but I don't like it when they are the first ingredient, and I try not to pick treats that are all grain.
for hard biscuit type treats, Old Mother Hubbard is nice. I really love Grandma Lucy's Organics if you can find them, They're cheap and really tasty (I'll admit, I've tried them, and I steal them from my dog- they're human grade, and taste like what they say they do) My dog loves them. Also most of the natural dog food brands make excellent treats- California Natural, Solid Gold, Innova, Canidae, Wellness etc.