Optional Garnish: crushed Oreo cookies or chocolate shavings
Place the Oreo cookies into a food processor and pulse until crumbly. Melt the butter and add it to the cookies. Pulse until this is combined.
Press this crust into a 9-inch pie plate, covering the bottom and going a little up the sides. You can use the bottom of a glass to press the crumbs down. Place the crust in the refrigerator while you are making the filling.
To make the filling, in a heavy large saucepan, whisk together the sugar, cornstarch, and cocoa.
In a separate large mixing bowl, whisk together the half and half and the egg yolks.
Gradually add this milk mixture to the saucepan, whisking constantly. When this mixture is smooth, turn the heat on to medium high. Whisk constantly as this begins to bubble.
Turn the heat down to medium and simmer this mixture for another minute. Make sure you are constantly scraping the bottom and sides. Don't turn your back on this.
Remove the saucepan from the heat. Add the butter, chocolate, and vanilla. Stir until thick and smooth.
Pour this mixture into the chilled crust. Place plastic wrap onto the pie and place in the refrigerator for at least 6 hours.
Once the pie has cooled, make the whipped cream. Place the cold heavy cream into a cold standing mixer bowl. Add the powdered sugar and vanilla. Beat this until peaks form.
Add the cornstarch and continue beating until the peaks are stiff.
Spread the whipped cream evenly on top of the chilled pie.
Top with crushed Oreo cookies or chocolate shavings, if desired.
This can be frozen, see above on how to do that.
Top this with crushed Oreos or chocolate shavings if desired.
You can use Cool Whip instead of making your own whipped cream.
This needs to chill for at least 6 hours to set up properly.
The half and half can be substituted, see above for other options.