Soak dried ceps in hot water for 5 mins. Strain ceps, keeping the liquid for the sauce. Squeeze ceps dry, then finely slice. Heat a large pan with unsalted butter until it just begins to foam, brown and bubble, add the button mushrooms, and the ceps. Sprinkle with salt if you want, then fry for 2-3 mins until lightly browned. Pour in the cep liquid and reduce until it becomes thick and syrupy. Stir in the double cream and boil for a min or so until sauce starts to thicken.