Preheat the oven to 350°F. Prepare the pie crust in a 9-inch pie pan, poke holes with a fork, and bake for about 20 minutes until lightly golden.
In a heavy-bottomed saucepan, whisk together the sugar, cornstarch, and salt. In a separate bowl, combine egg yolks, milk, and cream, then gradually whisk this into the dry mixture.
Cook over medium heat, stirring continuously, until the mixture bubbles and thickens, about 5–10 minutes. Whisk out any lumps.
Remove from heat and stir in the chocolate, butter, and vanilla until smooth and glossy.
Strain the custard through a fine mesh sieve into the baked crust to remove any lumps.
In a mixing bowl, whip the egg whites, salt, and sugar on high speed for 3–5 minutes until soft peaks form.
Spread the meringue evenly over the chocolate filling, sealing the edges. Bake uncovered for about 10 minutes until the peaks turn lightly golden.
Chill in the refrigerator for at least 6 hours before slicing and serving.