For the Brownie Batter: Gently melt butter and chocolate together in a glass bowl in the microwave or over a pot of simmering water. In a separate bowl whisk together eggs, salt, sugars and vanilla.
Stir egg mixture into chocolate mixture, and then add flour. Stir just until combined.
For the Chocolate Pudding: Combine 3/4 cup granulated sugar, salt, cocoa powder and flour in a medium bowl. Slowly stir milk into dry mixture.
Put mixture in medium sauce pot and cook until mixture begins to thicken.
Beat egg yolks in a small bowl, then briskly stir a small amount of the hot mixture (about 1/2 cup) into this mixture. Add egg yolk mixture back into the hot mixture in top of double boiler and stir in butter and vanilla. Cook until mixture thickens again. Set aside.
To Build Pie: Preheat oven to 350°. In a small bowl, combine the crushed chocolate sandwich cookies with the butter until thoroughly mixed. Press into a lightly greased springform pan. Bake for 15 minutes to set the crust.
Scrape brownie batter into the chocolate crust, forming an even layer. Bake for 20 minutes or until a toothpick comes out somewhat clean. Allow to cool slightly, and then pour pudding over the brownie layer, spreading into an even layer.
Combine the heavy cream and vanilla and whip until peaks form, and use to top the chocolate pudding layer. Garnish with more crushed chocolate sandwich cookies.