Cut a nice cooked calfs brain into thick slices, coat one side of each slice with a layer of Gratin forcemeat made by softening foie gras with a little cold brown sauce. Place a slice of truffle on top of each then put the slices together so as to re-form into the original shape of the brain.
Roll out some puff paste trimmings into a circle approximately 3 mm (⅛ in) thick and of a size proportionate to the brain; place the brain on the pastry to one side and coat with a little of the same forcemeat and sprinkle with a little chopped truffle. Moisten the edge of the paste and fold over the brain in the shape of a turnover; seal the edges together well and make a small opening in the top for steam to escape. Brush with beaten egg, allow to rest for 30 minutes then bake in a fairly hot oven for 15 minutes.
On removing from the oven, pour a few tablespoons of Sauce Perigueux into the opening in the top and serve on a folded serviette on a suitable dish.