Preheat oven to 325 degrees F. Separate egg yolks from whites; set whites aside for meringue. Place yolks in a small bowl; set aside.
For filling, in a medium saucepan, combine 1 cup sugar and cornstarch. Gradually stir in half-and-half and chocolate. Cook and stir over medium-high heat until thickened and bubbly; reduce heat. Cook and stir for 2 minutes more. Gradually stir about 1 cup of the hot mixture into yolks. Add egg yolk mixture to hot mixture in saucepan.
Bring to a gentle boil; reduce heat. Cook and stir for 2 minutes more. Remove from heat. Stir in butter and 1-1/2 teaspoons of the vanilla. Cover and keep filling warm.
For meringue, in a large mixing bowl, combine egg whites, remaining 1 teaspoon vanilla, and cream of tartar. Beat with an electric mixer on medium speed about 1 minute or until soft peaks form (tips curl). Gradually add 1/2 cup sugar, 1 tablespoon at a time, beating on high speed for five minutes more or until mixture forms stiff, glossy peaks (tips stand straight) and sugar dissolves.
Pour warm filling into baked pastry shell. Spread meringue over warm filling; seal to edge. Bake for 30 minutes. Cool on a wire rack for 1 hour. Chill for 3 to 6 hours before serving; cover for longer storage. Makes 8 servings.