1Mix together the graham cracker crumbs, sugar, and melted butter until evenly moistened. Press into a 9 inch pie plate and bake at 350 degrees F for 10-13 minutes, until lightly golden. Cool before filling.
Combine the cocoa powder and cornstarch in a large (3 quart) saucepan.
3Add the water and whisk until smooth. Whisk in the sweetened condensed milk and egg yolks until smooth.
4Place the saucepan on the stove over medium heat. Cook, whisking constantly, until mixture thickens (it should take about 5 minutes). When thick continue whisking for another 30 or 40 seconds,remove from the heat and whisk in the butter. When butter is melted, whisk in vanilla.
5Set a piece of plastic wrap over the surface and let cool for about 10 - 15 minute before pouring into the pie shell crust. Chill the pie for several hours.
6Top with whipped cream and some chocolate curls if desired.