For the caramel sauce, put the sugar in a saucepan with 2 tablespoons of water and heat until the sugar melts. Spread in the pot by swirling and let it caramelize golden.
In a second high saucepan, heat the cream and pour over the caramel. Be careful: it will boil vigorously bubbling.
Continue cooking the sauce until the caramel dissolves. Let cool a bit and stir 50g butter into the caramel sauce.