Preheat your oven to 350°F (175°C). In a medium bowl, combine the chocolate graham cracker crumbs, melted butter, sugar, and salt. Mix until the mixture resembles wet sand and holds together when pressed.
Press the mixture firmly into the bottom and up the sides of a 9-inch pie dish. Use the flat bottom of a measuring cup to compact the crust evenly.
Bake the crust for 8-10 minutes until it's set but not browning too much. Remove from the oven and set aside to cool slightly while preparing the filling.
In a microwave-safe bowl or using a double boiler, melt the 4 oz of semi-sweet chocolate until smooth. Set aside to cool slightly.
In a large mixing bowl, whisk together the pumpkin puree, eggs, heavy cream, and brown sugar until smooth.
Add the melted chocolate, pumpkin pie spice, vanilla extract, and salt. Whisk until fully incorporated and no chocolate streaks remain.
Pour the chocolate pumpkin filling into the pre-baked crust. If your crust edges are browning too quickly, cover them with aluminum foil or a pie shield.
Bake at 350°F for 50-55 minutes, until the center is almost set but still slightly jiggly (like gelatin). A knife inserted 1 inch from the edge should come out clean.
Transfer the pie to a wire rack and allow it to cool completely, about 2 hours at room temperature. For the best flavor and texture development, refrigerate overnight.
Just before serving, prepare the chocolate drizzle. Combine the 2 oz chocolate, 2 tablespoons heavy cream, and 1 tablespoon butter in a microwave-safe bowl. Heat in 20-second intervals, stirring between each, until smooth and glossy.
Allow the chocolate drizzle to cool slightly until it reaches drizzling consistency. Drizzle over the chilled pie using a piping bag or zip-top bag with a tiny corner snipped off.
Slice and serve with a dollop of whipped cream if desired.