Preheat oven to 325 degrees F. Mix 1 1/4 cups sugar with 6 Tbs. cornstarch, and salt in a medium saucepan. Add in 1/2 cup cold water, lemon zest, and lemon juice then whisk in the egg yolks. Add the butter and add 1 1/2 cups boiling water. Boil the mixture over medium-high heat and stir for about seven minutes. Reduce heat and stir an additional minute. Pour filling into already baked pie crust.

MERINGUE:

Beat egg whites and cream of tartar in a deep bowl with a hand mixer on high until very foamy. Continue to beat as you gradually add the sugar and cornstarch, one Tbs. at a time. Beat mixture until stiff peaks hold.

Spoon meringue on the top of the pie and use a spatula to cover the area of the pie. Bake in the oven for about 25 minutes, or until meringue is browned. Cool for 3 hours then serve.