I am from Piedmont and here, above all, once upon a time we used the burera, in Piedmontese dialect, in Italian as someone wrote it is called churn, many without this tool used a bottle which, shaken for a long time, divided the fatty part of the cream (butter) from the whey which was then drunk or used for cooking, all this was done some time ago, now both for convenience and for speed I use the planetary mixer or even electric whisks, the only ingredient is the cream, preferably fresh but it also comes with the one from the supermarket (although in my opinion it is useless because you might as well buy the butter already made) and proceed as for the whipped cream but leave the cream to swirl for a few more minutes, you will notice that the whey divides and the butter becomes compact, let the butter rest in its own whey for a few minutes after which divide it and immerse it in a basin of cold water and changing the water often, mash the butter making all the whey come out, you will understand that it is perfect when by immersing the butter the water remains transparent, and then give the shape according to the mold, any question please feel free