Boil the potatoes in salted water until they are 80% done. Peel them and let them cool. (This can be done the day before.)
Refrigerate the potatoes for at least 40 minutes or overnight.
Melt the sugar over low heat in a large, heavy saucepan. Do not touch the sugar until you see that the caramel is almost completely melted, then you can stir slowly with the help of a wooden spoon or a silicone spatula!
When the caramel has melted and browned completely, add the butter, stir and let the mixture boil until it becomes a caramel sauce. Add potatoes and gently stir them into the caramel. Let them simmer over low heat for 15-25 minutes, flipping them often, until they are coated with caramel on all sides.
When the caramelized potatoes are ready, sprinkle some fleur de sel on top of them and enjoy!