Spreading the meringue over warm filling helps it adhere. Place plastic wrap over the filling to keep it warm while you make the meringue. Use a knife dipped in hot water to slice the pie. The crust is a bit crumbly, but you won't mind after you taste the pie.
To prepare crust, combine the first 3 ingredients, tossing well. Press into the bottom and up sides of a 9-inch pie plate coated with cooking spray. Bake at 350º for 15 minutes or until lightly browned. Cool completely on a wire rack.
To prepare filling, combine 3/4 cup sugar, cornstarch, and dash of salt in a medium saucepan; whisk in milk and chocolate. Bring to a boil over medium heat, stirring constantly. Cook 1 minute, stirring constantly. Remove from heat. Gradually add 1/2 cup chocolate mixture to egg yolks, stirring constantly with a whisk. Return egg mixture to pan. Cook over medium heat until thick (about 4 minutes), stirring constantly. Stir in 1 teaspoon vanilla. Spread mixture into prepared crust. Cover surface of chocolate mixture with plastic wrap.
Reduce oven temperature to 325º.
To prepare meringue, place the egg whites, cream of tartar, and 1/4 teaspoon salt in a large bowl. Beat with a mixer at high speed until foamy. Add 1/2 cup sugar, 1 tablespoon at a time, beating until stiff peaks form. Add 1 teaspoon vanilla, beating just until blended. Remove plastic wrap from filling. Spread meringue evenly over filling, sealing to edge of crust.
Bake at 325º for 25 minutes. Cool 1 hour on a wire rack. Chill for 3 hours or until pie is set.