First, Make The Crust. Preheat the oven temperature to 350 degrees F. Line a metal 8x8 square pan with parchment paper and set aside. In a food processor, crush cookies until you reach fine crumbs. Add the melted butter and pulse to combine until the cookie crumbs are evenly coated in butter.
Shape & Bake The Crust. Pour the cookie crumbs out into the prepared baking pan and use the bottom of a measuring cup to pat the crust out into an even layer. Bake 5-8 minutes, or until fragrant, then set the crust aside on a wire cooling rack to cool while you make the pastry cream.
Start The Pastry Cream. In a large saucepan, combine granulated sugar, cocoa powder, and salt. Whisk until no clumps remain, then add heavy cream. Stir to combine well, then add the whole milk, and egg yolks. Mix well so no clumps of egg yolk remain.
Cook The Pastry Cream. Next, cook the pastry cream over medium low heat stirring constantly for 8-10 minutes, until it's thick enough to see the trail of the whisk in the pudding. Once the mixture starts bubbling, stir for only 5-10 seconds, then remove from the heat.
Stir In Chocolate, Butter, & Vanilla. Add the chopped chocolate, remaining butter, and the vanilla extract. Stir until the chocolate and butter is fully melted. At this point the mixture should look smooth and glossy. If you have any lumps, you can strain the mixture through a fine mesh sieve or use an immersion blender to puree until the pastry cream is completely smooth. Set the chocolate custard aside to cool 5-10 minutes, stirring occasionally as it cools.
Layer. Pour the chocolate pudding mixture over the cooled cookie crust and smooth into an even layer. (An offset spatula works great for this!) Chill the pie bars in the fridge for 1-1.5 hours until cool. Do NOT top with whipped cream until the chocolate filling has cooled!
Make The Chocolate Whipped Cream. When the pastry cream has cooled, start the whipped cream. In the bowl of a stand mixer (or a large bowl with a hand mixer), combine heavy cream, cocoa powder, powdered sugar, vanilla extract, and pinch salt. Beat on low speed with the whisk attachment for 30-60 seconds until the cocoa powder has mixed into the cream. Then increase the speed to high and mix 4-5 minutes, or until the whipped cream holds its shape. (This is also known as stiff peaks.)
Top & Garnish The Bars. Spoon the chocolate whipped cream on top of the cooled chocolate layer and spread into an even layer with an offset spatula. I like to make a few swirls here with the spatula. Sprinkle with more chopped chocolate or chocolate curls, if desired.
Chill, Serve & Store. If possible, chill the bars for an additional 30-60 minutes before cutting into slices and serving. To store leftover bars, cover the whole pan with plastic wrap or store individual bars in an airtight container in the fridge 3-4 days.