First, bake and cool an unfilled pie crust.
To make the lemon pie filling in a microwave…
In a large microwave safe glass bowl, mix 3 egg yolks (save the whites for the meringue topping), 1-1/4 cups of granulated sugar, about 2 teaspoons of grated lemon peel, 1/2 cup of lemon juice (which is about the juice of 3 lemons), 1/3 cup of cornstarch, and a dash of salt. Stir mixture with a wire whisk until well blended.
Then add 1-1/2 cups of very hot water to the above mixture – adding about a half cup at a time and then stirring after each addition.
When the mixture is blended together, microwave on High for about 2 minutes, and then stir with a wire whisk. Continue microwaving on High in additional 2 minute increments, until the mixture is thickened and pudding-like in consistency. (Since microwave cooking times vary, there is not a specific cooking time for the filling; you simply cook it until it is like pudding.)
After cooking, stir in 1 tablespoon of butter.
Spoon the lemon mixture into the pre-baked and cooled pie shell.
To make the meringue topping…
Mix 3 egg whites and 1/4 teaspoon of cream of tartar. Beat with electric mixer until soft peaks form. Then slowly add in 1/4 cup of sugar, a bit at a time, along with 1/2 teaspoon of vanilla. Continue beating until the meringue is glossy and stiff peaks form.
Spoon the meringue onto the top of the hot lemon filling, and spread evenly. With a knife, make small swirls in the meringue so it looks pretty when baked.
Bake the meringue-topped pie at 325 degrees for 20-35 minutes, or until it is golden brown and temperature (of meringue) reaches at least 160 degrees.
Cool for at least 2 hours before refrigerating.