In a medium bowl, combine the ground chocolate cookies and melted butter. Press the crust into the bottom of a 9-inch pie pan. Bake for 15 minutes, then allow it to cool slightly before filling.
In a medium bowl, whisk together the pumpkin puree, sweetened condensed milk, lemon juice, melted butter, brown sugar, eggs, vanilla extract, and nutmeg. Pour the filling into the prepared pie shell. Bake 55-70 minutes on the center rack, or until the filling is set, but is still slightly jiggly in the center.
Using a stand mixer or hand mixer, beat the heavy cream, powdered sugar, and vanilla extract until stiff peaks form. Top the pie with whipped cream and shaved chocolate.