Preheat your oven to 350°F. Poke holes in the unbaked pie crust with a fork and bake until lightly golden.
In a medium saucepan, whisk together the cocoa powder, 3/4 cup sugar, the flour, and 1/4 teaspoon salt with 2 lightly beaten egg yolks and 1 1/2 cups milk. Cook over medium heat, stirring constantly until the mixture thickens (about 5 to 10 minutes). It will not thicken further in the oven. Remove from heat and stir in the vanilla extract and butter.
In a clean bowl, beat 2 egg whites with a pinch of salt until soft peaks form. Gradually add the remaining 1/4 cup sugar and continue beating until stiff peaks form.
Pour the warm chocolate custard into the baked pie crust and top with the meringue, spreading it evenly and creating peaks. Bake at 350°F for about 8-12 minutes until the meringue peaks are lightly browned. Allow the pie to cool to room temperature before serving.