I've read (but have not tried) that it's possible to ferment cream with kefir grains, or yogurt, and use the resulting ferment to make cultured butter. There are creme fraiche recipes that are just cream and buttermilk also. For clarity: by buttermilk, I mean the fermented stuff that is found in stores, not just the leftover liquid from churning unfermented cream.

I've fermented milk with yogurt and crème fraiche but this was for something else - a type of cheese. If you're making butter you have to look at the fat content. Butter is milk fat. So you can directly calculate how much butter you can get from the starting product based on the fat %.

i've made cultured butter. you want to use the correct strain. i buy a buttermilk culture specifically