Place the flour, cocoa, sugar, and salt in a large mixing bowl and whisk to combine. Stir in the oil, until the mixture resembles coarse meal.
1 c. All-Purpose Flour, 1/4 c. Unsweetened Cocoa Powder, 2 tbsp. Sugar, 1/2 tsp. Salt
Add the butter pats, tossing to coat. Stir in the water (the colder the better), a tablespoon at a time, just until the dough gathers itself into a ball.
1/4 c. Butter, Unsalted, 3-5 tbsp. Water
Chill in the refrigerator for 1 hour.
On a lightly floured surface, pat or roll the dough into a rectangle about 1/2-inch thick. Fold it into thirds, like a letter. Roll out again to a thickness of 1/2-inch. Turn the dough sideways and repeat two more times.
Wrap the dough in plastic wrap and refrigerate for 2 hours minimum.
Dust the work surface with flour, and roll the dough out to an inch or two larger than your pie plate.
Roll the dough around the rolling pin and transfer to the dish. Trim off any excess, then roll and crimp the edge.
Bake in a preheated 375°F oven for 20 minutes. Side aside to cool completely.
Prepare the filling
In a large saucepan, whisk together sugar, cornstarch and cocoa. Whisk in milk and stir continuously over medium heat until mixture thickens and is bubbling.
1-1/2 c. Sugar, 6 tbsp. Cornstarch, 6 tbsp. Cocoa, 6 c. Whole Milk
Let mixture cool for 3-5 minutes and then pour into prepared pie crust.
Allow to cool at room temperature for one hour and then transfer to refrigerator to cool completely.
Make whipped cream while the pie cools
In a large bowl, whip cream until stiff peaks are just about to form.
1 c. Heavy Cream
Beat in vanilla and confectioners sugar until peaks form. Make sure not to over-beat or your whipped cream will then become lumpy and butter-like.