Preheat oven to 375 degrees. Lightly grease a pie plate.

In a medium-sized bowl, mix together flour, salt, and sugar. Cut the butter into small pieces and add to the flour mixture. Cut the butter pieces into the flour until the mixture is in pea-size pieces.

In a small bowl, combine egg yolk and water. Add egg mixture to the flour mixture, combine and form into a cohesive ball. Refrigerate for 20 minutes.

Remove from the refrigerator and turn out dough onto a clean and floured surface. Use a rolling pin or a sturdy glass to roll the dough into a circle with 1/8" thickness. Gently transfer pie crust to the pie plate, and press into the plate.

To bake: lay a piece of parchment paper onto the center of the pie crust. Use pie weights or dry beans to hold the parchment paper down. Bake for 15-20 minutes, or until edges are starting to brown. Remove from oven and allow to cool.

In a large saucepan, combine sugar, cocoa powder, flour, and salt. Add in evaporated milk and condensed milk and bring to a gentle boil over medium heat, stirring frequently.

Continue to boil an extra 2 - 3 minutes, stirring frequently, then remove from heat. Add in vanilla extract and stir.

Pour filling into pie crust, and refrigerate for 2 - 3 hours, until the filling is set.