Preheat the oven to 350 degrees.
In a medium bowl, whisk the 3 egg yolks until thick and lemon colored. Set aside.
In a medium saucepan, combine sugar, cocoa, salt, and cornstarch. Stir in about one cup of the milk.
Place the saucepan over medium high heat and with a large whisk, gradually add the remaining 2 cup milk, whisking constantly until well blended.
Bring to a low boil. Cook for about 1 minute, stirring constantly. Remove from heat.
Add a small amount of the hot mixture to the egg yolks, stirring well, then add the egg mixture back to the cocoa mixture in the saucepan.
Cook for about 2 minutes until the mixture begins to thicken, stirring constantly. Remove the pan from the heat and stir in the butter and vanilla.
Pour the chocolate mixture into the cooked, cooled pie shell. (Note: the chocolate mixture should be somewhat thickened but will finish setting up as the pie cools.)
Add the egg whites to a large mixing bowl. Beat on medium speed until foamy. Gradually add the ¼ cup sugar. Increase speed to high and continue to beat until stiff peaks form.
Spread the meringue immediately on the hot filling and bake for 12 to 15 minutes until golden brown on top.