Failed at something that's been done since the Middle Ages and don't k kw what I did wrong.

Tried to make the simplest thing ever - BUTTER

Put heavy cream in my mixer, added a little salt and watched it become whipped cream. Kept mixing waiting for it to become butter and then I was going to strain out the buttermilk, knead the butter and celebrate my brilliance.

Didn't work. I whipped it forever and all I seem to have gotten was "whipped salty cream." Not sure why but it never really separated.