Before you start: The secret to this desert is to have your ingredients measured and ready. It makes the process move along quickly. Prepare your favorite baked pastry shell. Cool on wire rack. Separate the egg yolks from the whites. Preheat the oven to 350 degrees.
Make custard: In a medium saucepan over medium heat, bring 2-1/2 cup of the milk almost to simmering (watch closely so milk doesn't boil). Meanwhile, in a medium bowl, with a 12-inch or larger balloon whisk, stir remaining 1/2 cup milk into the 3 egg yolks. In a second bowl, combine 1 cup sugar, the flour, cocoa powder and 1/2 teaspoon kosher salt; whisk in egg egg yolk mixture until smooth. Gradually whisk in hot milk; return the mixture to saucepan.
Bring to boiling: Over medium-high heat, cook and stir mixture until it comes to a full boil. You want large bubbles from the middle, and while you're whisking, be sure to get the whisk into the corners and weep the bottom so you won't have to worry about scorching -- which would require starting over! Boil for 30 seconds and remove from heat.
Fill pie shell: Whisk in chocolate and butter until melted and smooth. Stir in 2 teaspoons vanilla. Strain mixture through a sieve, pushing it through with a spatula as needed. There is a good chance there will be tiny pieces of cooked egg in your custard. Straining them out is a small, easy step that makes a big difference.
Prepare egg whites: Wash both bowl and whisk. Dry them with a clean towel. Any grease is going to give you trouble in a big way. Set whites - in bowl - over a bowl of hot (110 degrees F) water. You'll get more volume. Five minutes should do it. Keeping your elbow firmly against your side, whisk rapidly until white begin to mound. Move only your wrist. Whisk in superfine sugar, 2 tablespoons at a time, sprinkling the sugar over the whole bowl. Continue to whisk until whites are moist, glossy and do not slide when bowl is inverted. Plan on 7 or 8 minutes the first few time you try this. Tips of the whites should curl over slightly when whisk is lifted from the bowl.
Cover the pie with the meringue: Turn meringue out all at once on top of hot pie filling. With spatula, spread meringue from center to edges, making sure meringue seals to crust all the way around. Bake 15 minutes at 350 degrees or until top is golden. Ovens vary, and if you overbake, the meringue can weep.
Cool on wire rack for one hour. Refrigerate at least two hours before serving. Store leftovers (as IF!) in the refrigerator.
Note: If you don't want to whip the egg whites by hand, you can use an electric mixer on medium-high speed. :-).