Preheat oven to 350°F (175°C).

Prepare a double boiler.

Heat milk in the double boiler.

In a separate bowl, combine sugar, cocoa, salt, vanilla, and cornstarch.

Gradually whisk the dry ingredients into the heated milk, ensuring no lumps form.

Continue cooking and stirring the mixture until it thickens.

Remove from heat.

In a separate bowl, beat egg whites until stiff peaks form.

Gently fold the beaten egg whites into the cooked chocolate mixture.

Pour the mixture into the baked pie crust.

Bake in the preheated oven for 15 minutes, or until the filling is set.

Let the pie cool completely before serving.