I'm gobsmacked.
I thought you had to churn it in a turn handle thing for hours.
After seeing the video Gus posted (above), I thought I'd invest £1.40 from my local Tesco for 300ml of double cream and throw it in my cheap and cheerful Kenwood compact food processor which I use to make just about anything in. Honestly I was expecting to throw the lot in the bin as how can cream just turn into butter in a few minutes.
I didn't need any ice water or filter cloth, just about 6 mins in the mixer, it first went to a thin liquid, then went into thick cream and I was not expecting anything else. But its like magic, another couple of mins and white liquid starts splashing off and it turns yellow.
Put the lump into a sieve, poured the liquid away and washed the butter lump under the cold tap. Dried the water off with a sheet of kitchen roll, then put back into the mixer with 3g of salt and mix for anther minute or so.
Just under 200g of salted butter, mind blown.
...which freeze well.