4 eggs, separated
1 (14 oz.) can sweetened condensed milk
1/3 c. key lime juice
1/2 tsp. cream of tartar
1/3 c. sugar
1 baked 9-inch pastry shell

Combine egg yolks, condensed milk, and key lime juice in heavy saucepan. Cook over low heat, stirring constantly until mixture reaches 160 degrees, about 10 minutes.

Beat egg whites and cream of tartar at high speed on an electric mixer just until foamy. Gradually add sugar, 1 tablespoon at a time, beating until stiff peaks form and sugar dissolves, 2 to 4 minutes.

Pour hot filling into shell. Immediately spread meringue over filling, sealing to edge. Bake at 350 degrees for 25-28 minutes.