The "buttermilk" you get from making butter is not the same as the store bought buttermilk. The store buttermilk is made from a culture added to milk (at least in the US).
I didn't know this. I don't think they sell buttermilk (US kind) here in the UK, which is part of the reason I started down this path. It seemed like such a win/win, get butter and a starter for my sour cream.
Well, we had to chuck the ghee-like substance because I think it was starting to go rancid. I reckon you are absolutely right that it was cream underneath because we put the ghee in the fridge and the yellow part was solid but I was able to tunnel down to the white bit and drain off most of the white liquid. Further evidence that I just didn't mix it long enough.
However, we did get what I thought was buttermilk (probably less than a teaspoon) from the second attempt at butter and we added it to the little container of cream. I then left it out for maybe four hours, covered with a bit of kitchen towel. The directions I was following said it would take 12-18 hours but when we got back home mid-afternoon it was quite thick. We ordered Mexican takeaway for dinner and this sour cream was The Most Amazing sour cream I've ever had.
I'll never go back to ready-made. Don't know if it would have worked out the same without the buttermilk, but it seems like it probably would. I've heard you can make creme fraiche by just leaving a pot of cream out for however long it takes, and I believe sour cream is just the next step in the evolution of condiments.
Still want to have another go with butter, too. I'll use the Bamix on the salted stuff and see if it comes together, but I suspect you're right that the salt will prevent me getting positive results. It has still been so fun getting to know this whole process better. Thanks so much, and I will definitely check out that link you provided!