How to Make It
Preheat oven to 325°F. Whisk together first 3 ingredients in a medium-size heavy nonaluminum saucepan.
Whisk together egg yolks and next 2 ingredients in a large bowl; whisk into sugar mixture in pan over medium heat. Bring to a boil, and boil, whisking constantly, 1 minute. Remove from heat. Stir in butter and zest until smooth. Spoon into piecrust.
To make the meringue: Beat egg whites and cream of tartar at high speed with an electric mixer just until foamy. Gradually add sugar, 1 Tbsp. at a time, beating until stiff peaks form and sugar dissolves (2 to 4 minutes). Add vanilla, beating well. Makes enough for 1 (9-inch) pie.
Spread Mile-High Meringue over hot filling, sealing edges.
Bake at 325°F for 20 to 25 minutes or until meringue peaks are lightly browned. Let cool completely on a wire rack (about 1 hour). Store in refrigerator.