Granny's Famous Lemon Meringue Pie

Ingredients

• 1 pre-baked 9-inch pie crust (Granny's All-Purpose recipe)
• 1½ cups granulated sugar (divided)
• ¼ cup + 1 tbsp cornstarch (divided)
• ⅛ tsp salt
• 1½ cups cold water
• 6 large egg yolks
• 1 tbsp lemon zest
• ½ cup fresh lemon juice
• 2 tbsp unsalted butter
• ¼ tsp cream of tartar
• 4 large egg whites
• ½ tsp vanilla extract