Beat the first three ingredients together until thick and smooth. Pour mixture into pie crust.
In a large, clean, and dry mixing bowl, beat the egg whites on medium-high speed until frothy.
Gradually add the sugar, one tablespoon at a time, and beat the egg whites until stiff peaks form.
Add the cream of tartar and vanilla extract and beat until just combined.
Spoon the meringue onto the top of the pie filling, spread it evenly over the entire surface, and seal the edges against the pie dish.
Bake for 10 to 15 minutes at 350 degrees or until the meringue is golden brown.
Allow the pie to cool completely before slicing and serving.
Recipe Tips
To have fresh key lime juice all throughout the year, squeeze them while they're in season, freeze the juice in ice cube trays, and store the cubes in freezer bags.