Preparation

• Make lemon curd:
  Cut butter into pieces and in a heavy saucepan cook with zest, lemon juice, and sugar over moderate heat, stirring, until sugar is dissolved and mixture just comes to a simmer.
  In a bowl whisk together eggs and whisk in lemon mixture until combined well.
  Transfer lemon curd to pan and heat over moderate heat, whisking constantly, until it just begins to simmer. Pour lemon curd through a fine sieve into a bowl and cool slightly.
  Chill lemon curd, its surface covered with plastic wrap, at least 2 hours, or until cold, and up to 3 days.
  Fill tartlet shells in baking cups with lemon curd. Chill tartlets, covered, 1 hour. Preheat oven to 400°F.
  Make meringue:
  In a bowl with an electric mixer beat whites and cream of tartar until they hold soft peaks. Gradually add sugar, beating until meringue holds stiff peaks.
  Transfer meringue to a pastry bag fitted with 1/2-inch plain tip and pipe meringue 2 inches high onto each tartlet, completely covering lemon curd.
  Bake tartlets in middle of oven 3 minutes, or until meringue tips are just browned, and cool in cups on racks.
  Chill tartlets in airtight containers at least 2 hours, or until cold, and up to 1 day.
  Keep tartlets chilled until ready to serve.