Make the Chocolate Pastry:

1. Add the flour, cocoa powder, powdered sugar and salt to the bowl of a food processor. Pulse until combined.
2. Add in butter and pulse 7 times.
3. With the motor running add in the egg and let process until the dough forms together into a cohesive ball.
4. Scrape the dough from the bowl and place in a piece of plastic wrap and form into a disk and wrap tightly. Using a rolling pin roll the dough out until it stretches to the corner of the plastic wrap to ensure a tight seal.
5. Refrigerate for at least 2 hours, but preferably overnight.

To Make the Filling and Bake:

1. When ready to to make the pie, the preheat oven to 375ºF.
2. Roll out the pastry on a lightly floured surface to an 11″ circle. Transfer the pastry to a 9” metal pie plate (see note), crimping the edges as desired. Do not dock the pie dough. Place the pie pastry in the refrigerator while the filling is assembled.
3. Fill a small sturdy pot with at least 1 1/2 inches of water and turn on the heat to medium-high. This is the bottom of your makeshift double boiler.
4. Add the chopped chocolate, butter pieces and salt to a heat-proof bowl.
5. Set the bowl over the steaming water and melt the chocolate and butter together, stirring frequently. Once the chocolate has just nearly all melted, shut off the heat and remove the bowl from the double boiler (keep the pot of water on the stove as you will need it a little later).
6. To the melted chocolate add the sugar. Whisk until it is all combined. The chocolate might look like it’s separating, this is normal.
7. Add the eggs and egg yolks and whisk vigorously to combine until it comes together into a smooth filling.
8. Add in the cocoa powder and flour and gently whisk in. Then add in the evaporated milk.
9. Let the filling sit for about five minutes, and then strain the mixture twice through a fine-mesh strainer to remove excess air bubbles or clumps from the filling.
10. Pour the filling into the refrigerated chocolate pastry and bake for 30 minutes.
11. Lower the oven temperature to 350ºF and continue to bake until the pie is fully set up, about another 10-15 minutes. The crust should be just set up (it should not have a jiggly center). If at any point the edges of the crust appear to be cooking too much, cover with a pie cover or with tinfoil.
12. When the pie is done, remove from the oven and transfer to a wire rack to cool for 30 minutes.

To Make the Melty Marshmallow:

1. Return to the makeshift double boiler. If the water has evaporated, fill it back up. Turn the heat to medium-high.
2. Combine the egg whites, salt, sugar, and cream of tartar to a heat-proof bowl (the metal bowl from your electric mixer is perfect here) and whisk together until fully combined.
3. Set the bowl over the steaming water. Alternate between whisking the mixture and scraping down the sides of the bowl with a spatula, until egg whites reach a temperature at 160°F, roughly between 8 and 10 minutes, and all the sugar has dissolved.
4. Remove the mixture from the heat and attach the bowl to an electric stand mixer fitted with a whisk attachment. Turn the mixer on the lowest speed, and gradually increase the speed until it reaches medium-high speed.
5. Whip until the meringue is glossy and is beginning to pull away from the sides and the bowl, about 8 minutes. It should have stiff peaks and be cooled down significantly. Now add in the vanilla and whisk until combined.
6. When the pie is still warm, pipe on (or add with an offset spatula) the melty marshmallow topping. It can be optionally toasted with a kitchen torch.
7. Slice and serve warm.