Prepare and bake 1 piecrust shell. Cool.
For filling, in a medium saucepan, combine sugar, flour, and salt. Stir in cream and chocolate. Cook and stir until bubbly. Cook for 2 minutes more.
Stir a small amount of the hot mixture (about 1/2 cup) into yolks. Return all to saucepan. Cook for 2 minutes more; stir constantly. Remove from heat. Stir in butter and vanilla. Pour into shell.
Prepare Meringue (see recipe below). Top pie with meringue. Bake in a 350 degree F oven for 12 to 15 minutes or until golden. Store, covered, in the refrigerator. Makes 8 servings.
In a mixer bowl, beat egg whites and cream of tartar until high and frothy. Add sugar, a tablespoon at a time, beating after each addition until soft peaks form (tips curl). Add vanilla; beat for 1 minute more. Spread meringue over hot filling, carefully sealing edges. Using the back of a spoon, shape whites into peaks.