Using a mandoline, or sharp knife, slice the potatoes into very thin rounds. Grease a roughly 26cm round oven dish lightly with butter, then start placing the potatoes around the sides, overlapping them slightly as you go. Keep going, working your way into the centre of the dish. When you have a space roughly the size of your camembert left, place a similar size ovenproof dish, or ball of foil in its place. This will stop the potatoes from collapsing and make sure you still have room to place the camembert in the centre once ready. Tuck any remaining potato slices into any gaps you can see.