4 eggs, separated, plus one egg white
1 (14 oz.) can sweetened condensed milk
1/2 c. Key lime juice
1/2 tsp. cream of tartar
Green food coloring, if desired
1 (9 in.) graham cracker pie crust
1/4 c. sugar
Pinch of salt

Preheat oven to 325 degrees. Beat egg yolks vigorously, until thick and sunny yellow. Add sweetened milk and beat slowly; gradually adding lime juice and cream of tartar. If desired, add food coloring to make filling green. Spoon filling into crust. Bake until set, about 15 minutes. Remove from oven, let cool 20 to 30 minutes. Place in freezer and freeze for 3 hours before topping with meringue.