Combine sugar, cornstarch, and salt in medium sized saucepan. Stir in milk.
Set heat to medium/low and allow to come to a boil. Stirring constantly.Boil 1 minute and remove from heat.
Beat egg yolks on high with electric mixer.
Gradually pour about 1 cup of hot mixture into egg yolk while mixing. This prevents the eggs from cooking on contact with the hot liquid.
Add the egg mixture back into sauce pan. Return to the medium/low heat and cook 2-3 minutes, stirring constantly.
Remove from heat; stir in key lime zest and juice, and butter. Pour into baked pastry shell(s).
Meringue Instructions
Beat the 4 egg whites that are room temp and cream of tartar at high speed for about 1 minute. I use a Kitchen Aid mixer and the wire whisk attachment.
Gradually add in sugar and vanilla.
Beat just until soft peaks form 2-3 minutes. The time really depends on what mixer that is used.
Spoon meringue over hot filling until edges are sealed.
Bake in a 350 degree oven for 8-10 minutes until meringue is lightly brown.