Pre-bake pie crust according to package or recipe instructions.
Preheat oven to 375°F.
In a heavy-bottomed saucepan, whisk together all of the pie filling ingredients before placing the pan on the stovetop.
Once the ingredients are combined, turn on the heat to medium and heat mixture until it comes to a low boil. Simmer for 3 minutes, whisking constantly or scraping with a silicone spatula to ensure no mixture burns on the bottom of the pan.
While the mixture is cooking, whisk egg whites using a standing mixer until foamy/soft peaks form. Sprinkle ⅓ cup sugar and the cream of tartar over the egg whites and continue whisking until stiff peaks form.
Pour hot chocolate mixture into the prepared pie crust.
Immediately top with the meringue, using a clean spatula to spread the meringue and seal down to the crust all around the pie. Make little swoops in the meringue to form pretty peaks.
Bake the pie for approximately 5 minutes or until the meringue is toasted brown. Remove from the oven and allow pie to cool and set for at least 4 hours before slicing into it.
Notes
The sugar and cream of tartar help the egg whites to hold their stiffness (the sugar also adds a light sweetness.)As long as you mix all of your filling ingredients BEFORE heating, you can make the filling all in one step and do not have to temper the eggs or dissolve the cornstarch. If you want, you can whisk together the dry ingredients and combine the wet ingredients, and then pour them together, but I like to reduce dishes and do it all in one pan!Seal the meringue all the way to the edges of the crust to ensure that it doesn’t pull away from the crust or crack when cooling. Mounding the meringue over the HOT chocolate filling also helps the meringue to start cooking from the bottom up and prevents weeping/sweating.