Melt the chocolate: Place your bittersweet baking chocolate in a small saucepan and melt it gently over medium-low heat, or using a double boiler. Stir until completely smooth and glossy. Once melted, remove the mixture from the heat and set it aside to cool slightly — you want it warm but not hot when it’s mixed in later.
Cook the Egg/Sugar Mixture: In a small saucepan, whisk together the eggs, cane sugar, and vanilla. Cook over medium-low heat, whisking often, until the mixture reaches 160°F (use an instant-read thermometer). Remove from heat and let cool to room temperature (around 75°F).
Whip the Cream: In a large mixing bowl or stand mixer, beat 1 ⅓ cups heavy cream until stiff peaks form. Transfer to another bowl and refrigerate to keep cold. Wipe out the mixing bowl and switch to the paddle attachment. Cream the room-temperature butter until smooth and fluffy.
Make the Filling: Whisk the cooled, melted chocolate into the cooled egg mixture until combined. With the mixer on low speed, beat the chocolate mixture into the creamed butter until smooth and glossy. Gently fold in the whipped cream until fully incorporated and no streaks remain.
Assemble the Pie: Pour the filling into the baked and cooled Oreo crust. Smooth the top with a spatula. Cover loosely and refrigerate until set, at least 4 hours or up to overnight.
Whip the Topping: Just before serving, whip cold heavy cream with the whisk attachment until soft peaks form. Add maple syrup and vanilla, then continue whipping until medium peaks form.
Serve: Spread or pipe the whipped topping over the chilled pie. Garnish with chocolate shavings, curls, or a dusting of cocoa powder if desired. Slice and enjoy!