Keep the diced butter and ice water in the refrigerator until ready to use. Using a stand mixer bowl, weigh out the flour and add the salt. Mix using the paddle attachment on the lowest speed (stir) for about 10-seconds.
Add chilled diced butter to the bowl. On the lowest speed, turn the mixer on and off quickly for a few seconds to coat the butter with the flour. This helps to prevent the flour from spilling over. Continue to mix on low speed until the flour and butter have a mealy texture like wet sand and pea-sized pieces evenly throughout, about 60 to 70-seconds. Do not overmix. The dough should not bind together before the water is added. Alternatively, use a dough/pastry blender or your fingers to break the butter into the dough.
Gradually add 1 tablespoon of ice-cold water to the bowl. After each addition, turn the mixer on for 2 to 3-seconds. Only add enough water until the dough looks lumpy and hydrated, but not wet or sticky. It should begin to clump together with small crumbles on the bottom of the bowl. All of the water may not be needed, about 5 to 7 tablespoons is typical. When the dough is pinched together, it should compress and hold, not be dry or crumbly. Do not over mix. The dough will be pressed together before resting.
Press the dough into a 1” thick round disc and wrap it in plastic wrap and place inside a resealable bag. Store in the refrigerator to rest for at least 4-hours, overnight, or up to 2 days.
Once removed from the refrigerator, allow the crust to sit at room temperature for about 5 to 10-minutes. This will make it easier to roll. If it’s still too hard, let it sit at room temperature until more pliable.
Dust the counter and top of the dough with flour. When rolling out, make sure to rotate and dust with flour underneath and on top. This will prevent the dough from sticking and make it easier to transfer to the pie dish. Roll the dough into a 14-inch circle, slightly less than ¼-inch thick.
Place the dough into a 9-inch pie dish and gently press against the sides and bottom. With a paring knife, trim the excess dough with a ½-inch overhang. Tuck the excess underneath the bottom crust edges. Crimp by pinching the dough using the pointer and thumb fingers. Place the crust in the freezer for 20 minutes.
Place the oven rack in the center position. Preheat to 375°F (190°C). Place the pie crust on a sheet pan. Place a piece of parchment paper in the pie dish and add the pie weights to cover just the bottom and sides of the crust, do not overfill.
Bake the crust for 20 minutes. Remove from the oven, and allow the weight to sit in the crust for a few minutes to press down any puffed up crust. Carefully lift the parchment paper filled with weights out of the pie dish and set aside, it will not be used again.
Bake the crust for 15 minutes. Check to see if the bottom and sides have shrunk slightly. Use a spoon to gently press the bottom and sides, don’t force it too much.
Bake the crust until golden brown and dry, about 10 to 15 minutes. Transfer to a wire rack and until completely cooled, 30 to 40 minutes.
In a small bowl whisk together sugar, cornstarch, cocoa powder, and salt, set aside. In a medium bowl whisk together the whole eggs and egg yolk. Add the cornstarch mixture to the eggs, whisk until combined.
In a medium pot add the milk. Heat over medium-low heat until it reaches 180 to 185ºF (82 to 88ºC), it should not be bubbling, about 7 to 8 minutes. Do not boil the milk, or it will curdle or scorch. Turn off the heat. Take the pot off the heat and place it on a towel beside the bowl with the egg mixture.
Slowly whisk in ¾ cups of the hot milk into the egg mixture. Whisk vigorously after each addition. This process tempers the eggs, bringing two different temperatures together without curdling. Start whisking the pot of milk and slowly pour the bowl of the tempered eggs into the pot. Heat over medium heat, whisk constantly and vigorously until the custard thickens, about 2 to 3 minutes. There may be some small lumps in the custard. Turn off the heat and whisk in the chopped chocolate, softened butter, and vanilla extract.
Strain the chocolate custard through a fine-mesh strainer into a bowl, use a spatula to press through. This will remove any lumps. There should be about 3 ¼ cups of custard.
Carefully pour the custard into the prepared pie shell. Use a spatula to smooth out the surface. Immediately put a piece of plastic wrap directly on top to prevent any film from forming. Place the pie in the refrigerator and let chill for at least 3 hours or overnight, this firms up the filling, making it easier to slice. It can also be chilled in the freezer for 1 ½ to 2 hours before adding the whipped cream.
In a stand mixer fitted with the whisk attachment, add heavy cream, powdered sugar, and vanilla to the bowl. Alternatively, if using a hand mixer use the whisk or beaters attachment and a large bowl, or whip by hand, whipping times may vary.
Whip the mixture on medium-low speed (setting 4) until the cream is frothy with bubbles on the surface, 1 minute. Increase the speed to medium-high (setting 8), and whip until a fluffy and smooth consistency is reached, about 1 minute 15 seconds for soft peaks. Continue to whip in 5-second intervals until stiff peaks form. Be careful, the consistency will change quickly.
Use a spatula to spread the whipped cream over the entire pie for a rustic appearance. Be careful not to overspread the cream as more agitation can cause a texture change. Alternatively, transfer whipped cream to a piping bag fitted with the desired tip to make a design. Garnish with chocolate shavings if desired. Refrigerate the pie until ready to serve.
Chocolate Garnish: Use a peeler to create chocolate shavings with a chocolate bar. I like to use semi-sweet or 60% dark chocolate.
Storing: Loosely cover the pie with foil or transfer leftover slices to an airtight container for up to 3 days.