Prepare the leeks: Line four 3-inch custard cups liberally with softened butter. Refrigerate. Trim roots from leeks. Trim the tough, dark green tops of the leeks. Split the leeks in half lengthwise. Wash thoroughly. Cut the leeks into 5 inch long by 1/4 inch wide strips. Separate the light and dark pieces. Bring a medium saucepan of salted water to the boil. Have a bowl of ice water ready on the side. Plunge light pieces of leek in the water for 1 minute. Refresh in the ice water. Remove and pat dry with paper towel. Repeat procedure with dark pieces of leek. Line the custard cups with alternating light and dark strip of leek by place one end of each strip in the center of the custard cup, pressing it along the bottom and up the side. There should be an overhang of approximately 1 1/2 inches over the edge of the cup. Refrigerate. Reserve remaining light pieces of leek for mousse.

Prepare the mousse: Bring a medium saucepan of salted water to the boil. Have a boil of ice water ready on the side. Plunge the asparagus into the water for 2 minutes. Refresh in ice water. Trim heads of asparagus and reserve for garnish. Place asparagus stems, reserved extra pieces of leek, chicken breast, egg, yolk, heavy cream and milk in a food processor. Puree until smooth. Season with salt and pepper. Press mixture through a fine sieve using a rubber spatula to remove fibers. Divide mixture between the custard cups. Fold leek ends over mousse to enclose it. Cover the top of the custard cup tightly with plastic wrap. Refrigerate until needed. When ready to cook, prepare a water bath: Place custard cups in a baking dish. Fill baking dish with hot water that reaches 3/4 of the way up the sides of the cups. Cover dish with aluminum foil. Place in a preheated 350 degree oven for 20 minutes. The mousse is cooked when a toothpick inserted in the center of a custard cup comes out completely clean. Remove from the water bath.

Make the sauce: Bring vegetable stock to the boil. Reduce by half. Add heavy cream, lemon juice, butter and chervil sprigs. Blend mixture together. Strain through a sieve. Keep sauce warm in a small saucepan.

Assemble the dish: Remove plastic from the custard cup. Unmold the chartreuse onto a serving plate. Heat reserved asparagus heads in the sauce. Spoon 3 heads of asparagus and some sauce on each plate. Sprinkle with truffle pieces and chervil flowers. Optional garnish: Sauteed wild mushrooms, such as chanterelles, morels and girolles.