Bake phyllo shells at 350° for 5 minutes; set aside.
Heat lime juice in a small saucepan until hot.
Beat egg yolk with a mixer at medium speed 2 minutes. Gradually add lime juice, beating at low speed until blended. Return egg mixture to saucepan; cook over low heat, stirring constantly, 5 to 6 minutes or until thick and pale yellow. Stir in milk and lime rind.
Spoon filling evenly into phyllo shells. Cover and chill 1 hour.
Top each tartlet with 1 teaspoon whipped topping. Garnish with lime zest and mint sprigs, if desired.