Skim the cream off it, whip the cream into whipped-cream, continue whipping it into very stiff whipped-cream, and then just DON'T STOP whipping it, and it'll soon start clotting into butter; no souring at all required.
I know this for a fact, 'coz we did exactly that as an experiment in Home Ec in the eighth grade. Thank GRR the electric whick was invented by then!
The liquid that remains, though, is a lot thinner than sour milk; just lightly white-coloured water, IIRC. (No guarantees I do, though; we didn't focus on that part AFAICR.)