Preheat the oven to 350°F. Gather and prepare all ingredients.
Crush the cookies in a food processor or ziplock bag until finely crumbled. Fold in 4 Tbsp melted butter until combined.
Gently push the crushed cookie mixture into the bottom of a 9-inch pie pan, pressing down and up all sides. I like to use a measuring cup to push along the edges. Bake in the oven for 10 minutes until set. Remove it from the oven and let it cool completely for at least 30 minutes.**
While cooling, make the brownie batter. In a medium bowl, add the 4 tablespoons of softened butter, ½ cup of sugar, egg, and vanilla extract. Cream with a rubber spatula until combined. Next, add the ¼ cup cocoa powder, flour, and baking powder. Fold until all the ingredients are combined.
Carefully spoon the brownie batter on top of the cooled crust, and spread it into an even layer. Cook for 25-30 minutes until a toothpick inserted comes out clean. Turn off your oven. Allow to cool completely again, at least an hour.**
While cooling (again), make the chocolate pudding. In a medium saucepan, add the ½ cup sugar, ⅓ cup cocoa powder, and cornstarch to a small sauce pot and stir to combine.
Add the milk and vanilla extract to the sauce pot. Place the pot over medium heat and whisk everything together. Continue to heat the mixture while whisking until it comes to a gentle simmer, at which point it will thicken into a pudding. (This will take some time; be patient.) Remove the pudding from the heat.
Once thick, pour and spread the chocolate pudding evenly over the cooled brownie layer of the pie. Press a layer of plastic wrap on the surface of the pudding to prevent a skin from forming on top. Place in the refrigerator to cool completely, at least 2 hours, or overnight.**
After the pudding has cooled, make the whipped cream. In a medium bowl, add the heavy cream. Using a hand mixer, mix at high speed until thick and fluffy, but not stiff. Turn the hand mixer to low, add the powdered sugar, and continue beating until incorporated. Then, turn the mixer to high until stiff peaks form.
To finish the pie, spread the whipped cream on top of the cooled chocolate pudding layer. Garnish with chopped chocolate chips and enjoy.
*Store-brand chocolate sandwich cookies are just as delicious but even cheaper than the name-brand Oreos! Go for the store's own-brand cookies to save money.**The secret is to let each layer cool as much as possible before moving on to the next layer. I like to break this pie up into two days and place it in the refrigerator overnight after the pudding layer is added. For this reason, the total prep and cook time does not include the cooling time, as this will vary depending on how long you allow each layer to cool.