Preheat your oven to 375°F (190°C). This is essential for ensuring your pie crust bakes evenly and achieves that perfect golden color.
On a lightly floured surface, roll out the pie dough to about a 12-inch circle. The dough should be thin but not so thin that it tears easily. Once rolled, gently roll the dough around the rolling pin and carefully transfer it to a 9-inch pie dish.
Gently press the dough into the dish, ensuring it fits well into the edges. Trim any excess dough from the edges to achieve a clean look. Fold over the top part of the dough (about a ½ inch). Use your fingers to crimp the edges, giving it that classic pie appearance.
Place the pie dish in the refrigerator for 1 hour, or in the freezer for 15 minutes. This helps the crust to chill and prevent shrinking during baking.
While the crust is chilling, prepare to bake it. Preheat your oven to 375°F again if it’s cooled down.
Line the chilled pie crust with parchment paper, then fill it with pie weights, dried beans, or uncooked rice. This will help keep the crust in shape as it bakes. Press into the bottom edges to avoid bubbling.
Place the crust in the oven for 25 minutes. After that time, remove it and carefully take out the weights and parchment paper. Return the crust to the oven and bake for another 10 minutes, until it’s lightly golden. Set aside to cool.
In a large bowl, or in the bowl of your stand mixer, add the room-temperature egg whites and cream of tartar. Use a hand mixer (or your stand mixer) on medium speed to mix the whites with the cream of tartar until they’re combined and frothy.
Add the sugar (½ cup) gradually while increasing the speed to HIGH. Continue mixing on HIGH until stiff peaks form, which can take anywhere from 2 to 8 minutes depending on your mixer’s strength.
Set the meringue aside and leave the oven on. Now, let’s prepare the chocolate filling. In a medium-sized saucepan, whisk together the sugar (¾ cup), cornstarch, and salt.
Whisk in the whole milk until fully incorporated and the dry ingredients dissolve completely.
Over medium heat, bring this mixture to a simmer, which should take about 4 to 5 minutes. You’ll notice it starting to thicken.
Once thickened, turn off the heat and stir in the unsweetened chocolate until fully melted. This should create a smooth consistency. Then, stir in the egg yolks until you no longer see streaks.
Turn the heat back on to medium and bring to a very gentle simmer again. Make sure to scrape any chocolate from the sides of the pan as you stir. Simmer for another 2 minutes.
Turn off the heat and stir in the unsalted butter and vanilla, mixing until the butter has melted and is incorporated.
Now, pour the hot chocolate filling into the prepared pie crust. Immediately spoon the meringue over the hot filling, spreading it to the edges of the crust. Ensure the meringue touches the crust to prevent shrinking.
Place the pie in the oven for about 10 to 15 minutes, until the tips of the meringue turn a lovely golden brown. Remove the pie and let it cool on a baking rack on the counter for 1 hour.
Finally, chill the pie in the refrigerator for at least 2 to 3 hours before slicing and serving. To slice, dip a sharp knife in a tall glass of hot tap water to ensure clean cuts.