Prepare your brownies according to this recipe or your favorite brownie mix—baking them for 30 minutes in a well-greased pie plate. Allow to cool completely.
Once the brownie crust has cooled, begin to make the filling. Melt chocolate and cool slightly. In a stand mixer using the paddle attachment, cream together the butter and sugar until light (about 2 minutes).
Scrape down the side of the bowl, and add the melted chocolate and vanilla. Blend well and then switch to the whisk attachment.
Add the cold eggs one at a time, whisking 5 minutes after each addition. This will take you a total of 20 minutes for the 4 eggs.
Spread the filling onto your cooled brownie pie crust and chill 4 hours (overnight is best).
Before serving, whisk your cream with the powdered sugar until you have stiff peaks, spread on the pie and top with some extra shaved chocolate.