In saucepan (off the heat), combine the cornstarch, sugar, and salt; slowly add some of the milk to make a paste.
Stir in remaining milk and whisk in egg yolks
Break chocolate in pieces and add to mixture
Place saucepan on medium heat and cook, stirring constantly, for 10 to 15 minutes until custard thickens and the chocolate melts.
Remove from heat and stir in vanilla; pour into prebaked 10” pie shell.
With an electric mixer, whip egg whites and cream of tartar at high speed until soft peaks form
Add the vanilla and sugar; beat until glossy, stiff peaks form
Spread the meringue over the chocolate filling, making certain it touches the edges of the crust all around.
Bake for 12 to 15 minutes until meringue is gold brown