Recipe:

1 cup white sugar
5 tablespoons unsweetened cocoa powder
2 eggs
1 (5 ounce) can evaporated milk
1/4 cup melted butter
1 teaspoon vanilla extract
1 (9 inch) unbaked pie crust (My favorite recipe for it is http://www.seriouseats.com/recipes/.....gh-recipe.html however a store-bought crust is fine if that intimidates you)

Preheat oven to 350 degrees F (175 degrees C).

*** NOTE *** I didn't have problems getting this to set cooking as directed below; you can temper the eggs with some addendum instructions below the main ones if you'd like, it will taste the same and is guaranteed to set, though you want to check the pie at 45 minutes baking time if you do this.

Mix sugar and cocoa together.
Beat the eggs then add to the cocoa mixture.
Beat in the milk, butter and vanilla.
Pour mixture into 9 inch unbaked pie shell and bake at 350 degrees F (175 degrees C) for 45-60 minutes or until set.
Let cool before slicing.

Tempering instructions if your pie doesn't set:
Mix all ingredients (EXCEPT the eggs) and stir over medium heat until the chocolate is blended.
Remove from heat.
In a separate bowl stir the eggs, while stirring the eggs, slowly add 1/2 cup of the hot pie mixture to the eggs to gradually even the egg temperature.
Now pour the egg mixture into the remaining pie mixture, continue to stir, It starts getting thicker almost immediately.
Pour into crust and bake.