Boil the potatoes, drain really well, then mash. Chill completely. Add the cream, a large knob of butter and a good pinch of salt. Beat the mixture together with electric beaters until it is as light as you can get it. Chill again. Add enough flour to stiffen the mix then knead until smooth. Take pieces about the size of an egg and gently roll each one out until it is quite thin (about ½cm at the most).