Assemble all the ingredients – especially meringue ingredients. By this I mean have the egg whites in the mixing bowl and the cream of tartar, sugar and cornstarch measured.
In a 2- to 3-quart sauce pan that is not yet set over heat, mix 1 1/4 cups sugar with 6 tablespoons cornstarch and salt. Whisk in 1/2 cup cold water, lemon zest, and lemon juice, blending thoroughly. Whisk in the egg yolks. Add the butter and whisk in 1 1/2 cups boiling water.
Set pan over medium or medium high heat and stir with a flexible heat-proof spatula scraper or wooden spoon until mixture boils, about 5-7 minutes. Reduce heat to medium and stir filling 1 minute more. Pour filling into pie crust and quickly make meringue.
In bowl of a stand mixer using whisk attachment, beat egg whites and cream of tartar until very foamy. Continue to beat at high speed and gradually add the sugar and cornstarch mixture, 1 tablespoon at a time. Then beat until whites hold stiff, glossy peaks. Spoon meringue onto hot pie filling. With a spatula, swirl meringue to cover filling completely, touching the rim of the pie shell.
Bake in a 325 degree oven until meringue is browned, about 25 minutes. Set pie on rack until cool, about 3 hours. Serve immediately or store in a cake cover in refrigerator.