Place sugar, lemon zest and eggs in a heavy-bottomed sauce pan; set over medium heat. Whisk constantly until mixture is combined and warmed through, about 3 to 4 minutes. Add lemon juice and butter; cook, whisking constantly, until slightly thickened and bubbly, about 4 to 5 minutes. Cool to room temperature and then refrigerate until thoroughly chilled and thickened; once chilled, fold in whipped topping until just combined.

Preheat oven to 350°F.

Place prebaked phyllo shells on a cookie sheet; bake until crispy, about 5 minutes. Let cool and then spoon about 2 teaspoons lemon mixture into each shell. Yields 1 tartlet per serving.