Open and sterilize 8 eggshells:
Carefully remove and discard top third of each eggshell by tapping around egg with a knife, then gently prying off top. Reserve 3 yolks in a bowl for lemon curd. If making trompe l'oeil toasts, reserve 3 whole eggs in another bowl. Reserve remaining whites and yolks for another use.

With a small knife, tear any membrane remaining in bottoms of eggshells. Cover shells with 2 inches cold water in a 3-quart saucepan and add vinegar. Simmer over moderate heat, gently stirring occasionally and skimming off any debris, 15 minutes. Carefully transfer shells with a slotted spoon to a rack to cool. Wipe shells inside and out gently with a dampened paper towel if any egg remains.

Make lemon pudding:
Whisk together cornstarch, xylitol, and a pinch of salt in a 1 1/2-quart heavy saucepan, then whisk in half-and-half until smooth. Bring to a boil over moderate heat, whisking constantly, then boil, whisking constantly, 2 minutes. Remove from heat and whisk in lemon juice and butter, then transfer pudding to a metal bowl. Set bowl in a larger bowl of ice and cold water and stir frequently until cold, about 10 minutes. Transfer pudding, covered, to refrigerator (reserve ice water).

Make lemon curd:
Whisk together zest, lemon juice, sugar, and yolks in a 1- to 1 1/2-quart heavy saucepan. Add butter and cook over moderately low heat, whisking frequently, until curd is thick enough to hold marks of whisk and first bubbles appear on surface, about 4 minutes.

Force lemon curd through a fine-mesh sieve into another metal bowl, scraping bottom of sieve, then transfer to ice bath and stir frequently until cold. Cover surface of curd with wax paper and chill in refrigerator until ready to serve.

Assemble eggs:
Arrange eggshells in an empty egg carton or eggcups. Spoon about 2 tablespoons lemon pudding into each eggshell and make a small well in center. Transfer lemon curd to a plastic bag, pressing out excess air. Snip off 1 corner of bag to create a 1/4-inch opening. Twisting bag firmly just above curd, pipe about 1 tablespoon into center of each egg to form "yolk." Serve puddings in shells with spoons, and with trompe l'oeil toasts if making.

Cooks' notes:·Pudding and curd can be made 1 day ahead and chilled, covered. (Keep sterilized eggshells in an airtight container at room temperature.) Whisk before using.
·Eggs reserved from emptied shells should be chilled, covered, and used within 2 days.