Chocolate Meringue Pie

In the saucepan off the heat, combine the cornstarch, sugar and salt. Slowly add the milk, stirring to make a paste. Stir in the remaining milk and whisk in the egg yolks. Break the chocolate into pieces and add to the milk. Place the saucepan on medium heat and cook, stirring constantly, for 10 to 15 minutes, until the custard thickens and the chocolate melts. (Have patience--just when you're sure the chocolate will be a speckled mess forever, it reaches the right temperature and instantly becomes silky and smooth.) Remove from the heat, stir in the vanilla, and pour into the prebaked pie shell. Set aside. Preheat oven to 350 degrees. With an electric mixer, whip the egg whites and cream of tartar on high speed until soft peaks form. Add the vanilla and sugar and beat until glossy, stiff peaks form. Spread the meringue over the chocolate filling, making certain it touches the edges of the crust all around. (The baked meringue will shrink if it is not spread out to the crust's edge.) Bake for 12 to 15 minutes, until the meringue is golden brown. Cool to room temperature, then chill before serving.