Place 1 potato on each piece of foil, add a little butter and 1 sprig of rosemary, wrap in the foil. Place on a baking tray with the seal facing upwards.
Approx. 50 mins. in the centre of an oven preheated to 200°C. Insert a sharp knife into the middle of the largest potato. If the knife comes out without any real resistance, then the potato is cooked.