OREO COOKIE CRUST: Add Oreo cookies to a food processor; pulse into fine crumbs. Pour in the melted butter, process until combined.
Press the cookie crumb mixture evenly and firmly into the bottom and up the sides of a deep 9-inch pie dish or springform pan with a measuring cup. Bake for 8-10 minutes; set aside to cool.
CHOCOLATE FILLING: In a large saucepan, whisk together sugar, cornstarch and cocoa. Whisk in milk until well combined. Place the saucepan over medium heat; cook whisking constantly, until the mixture is thickened and bubbling over entire surface about 8-10 minutes. Remove from heat. Add chocolate, butter, and vanilla, whisk until the chocolate is melted and filling is smooth.
Pour filling into the cooled cookie crust and smooth into an even layer. Place a piece of plastic wrap gently on top and refrigerate until filling is set, about 4–6 hours preferably overnight.
WHIPPED CREAM: In a large bowl, using an electric hand mixer, beat heavy cream on high speed for 1-2 minutes.
Add the sugar and vanilla extract; continue beating until soft peaks form, about 2-3 minutes.
Spread the whipped cream over the pie and garnish with curled chocolate, if desired.