Preheat oven to 400 degree F and arrange rack in lower third. Unfold 1 piecrust on a lightly floured surface. Using a floured rolling pin, roll out onto a 12-1/2-inch round. Using a 5-inch round cutter, cut out 4 rounds. Repeat with remaining piecrust to make a total of 8 rounds, for shells.

Fit each shell into a 4-inch round removable-rim fluted tart pan, allowing shell to extend up past edge of pans. Prick bottoms and sides of shells several times with a fork and chill until firm, 15 minutes. Bake on a cookie sheet until shells are golden brown, 12 to 15 minutes. Transfer in pans to a rack and cool completely.

Meanwhile, whisk together zest, juice, sugar, eggs, yolks, and salt in a heavy 2-quart saucepan. Add butter and cook over moderately low heat, whisking constantly, until butter is melted, curd is thick enough to hold traces of the whisk and just begins to simmer, about 15 minutes.

Using a spoon, push curd through a fine-mesh sieve set over a bowl to remove lumps. Cover surface of curd with waxed paper and let cool. Spoon into shells and chill, loosely covered, until curd is set, 4 hours or overnight. Garnish with whipped cream and flowers, if desired. Makes 8 servings.