Butter oil is, in my understanding, the fats from butter after everything else has been removed. In that sense it's not unlike clarified butter, but probably made with a centrifuge rather than heat. Adding milk powder (and presumably some water) sounds like basically adding back in what got removed to make the butter oil. I would hazard a guess that this is done because both butter oil and milk powder separately have far longer shelf lives than butter does
So I think that it's basically the same components as regular butter, they've just been separated out and then recombined. I have no idea if this does anything to the flavour
Might also be to get a consistent product , fat percentages probably vary, by taking it apart and putting always the same amounts together again they can always make it taste the same. I believe they do the same with orange juice
Just start making your own. It's actually really really easy.
Required equipment:
Big mixing bowl
Electric mixer
Required ingredients:
Extra thick double cream/whipping cream
Optional ingredients:
Salt
Garlic
Wild garlic
Insert herb here
Process:
place cream in bowl.
whip until it separates, folding the chunks back in and stop when there's only butter and
decant newly made buttermilk.
wash butter to remove buttermilk by filling bowl part way with cold water and just squeezing the butter. Switch water and wash until no visible milkyness comes out of the butter.
add extras by folding or blending it through the butter.
store any butter you're not likely to use within a few days in the freezer, I like to portion it out into 100g bits, so I know I won't be wasting any of it.
There, now you'll never have to wonder what your butter is made from again!
I would like to say that while this is clearly made in jest, unsalted butter is a requirement for some really great recipes, and also some people are on say a low sodium diet. I put it as optional, because I'm a mature person and don't yuck other people's yum.
Unsalted butter should be used when cooking specifically because you can control the salt level yourself directly by... Adding salt. It's easy to add salt, but very difficult to rebalance a dish when something is too salty.
Salted butter should be used when you're adding it to something that's already done, like when buttering toast.