Combine 4 ounces of chopped bittersweet chocolate and 4 tablespoons of butter in a heat safe bowl and warm using the microwave or a double boiler until the butter has melted
Mix until the chocolate and butter is completely combined and set aside to cool slightly
In a large bowl, whisk 1 large egg, a large egg yolk, a teaspoon of vanilla extract and 1/2 cup of brown sugar until fully combined
Add the cooled butter and chocolate mixture and mix until combined
Add 1/3 cup of all purpose flour, 3 tablespoons of unsweetened natural cocoa powder, 1 tablespoon of espresso powder (optional) and 1/4 teaspoon of salt and mix until just combined
Pour into the par baked pie crust and bake for 26-28 minutes in a 350° oven (cover the exposed pie crust if browning too much)
Transfer the pie onto a cooling rack while you prepare the pecan topping