1 1/2 c. firmly packed, coarse shortbread cookie crumbs
1/4 c. (1/2 stick) unsalted butter, melted
4 eggs, separated
1 (14 oz.) can sweetened condensed (not evaporated) milk
1/3 c. key lime juice
1 tsp. grated lime peel
Green food coloring, a few drops
Whipped topping

Mix cookie crumbs and melted butter together and form crust in pie pan. In large bowl, beat egg whites until stiff; set aside. In small bowl, beat egg yolks, add sweetened condensed milk. Gradually add lime juice and lime peel. Stir well and fold mixture into egg whites; add food coloring. Pour mixture into crust and freeze 3 hours. Let stand 15 minutes at room temperature. Serve with whipped topping and a slice of lime.