Combine sugar and cornstarch in top of a double boiler, stirring well. Add egg yolks, butter, lemon rind and juice, salt, and water; beat with a wire whisk. Cook over boiling water until smooth and thickened, beating constantly with wire whisk. Remove from heat. Spoon mixture into tart shells.
Beat egg whites (at room temperature) until foamy. Gradually add powdered sugar, 1 tablespoon at a time, until stiff peaks form. Spread meringue over filling of each tart, sealing to edge of pastry. Bake at 350° for 6 minutes or until meringue is lightly browned. Cool before serving.