Place the Oreo cookies in the food processor and pulse until you have fine crumbs. Transfer the crumbs into a mixing bowl, add melted butter and mix well.
Put the Oreo mixture into a 9-inch tart pan and spread it evenly on the bottom and up the sides. Press all over, packing it down. Refrigerate until firm, about 30 minutes.
Beat the eggs, sugar, and cornstarch together in a medium mixing bowl. Set aside.
In a medium saucepan, warm the milk and chocolate over medium heat until the chocolate is melted, stirring frequently.
To temper the egg mixture, slowly pour about one cup of hot chocolate milk into the egg mixture while whisking vigorously. (This will prevent the eggs from scrambling in the hot milk.)
Pour the tempered egg mixture back into the pan with the remaining milk. Cook the pudding mixture until it thickens, stirring constantly on medium-low heat, about 7-8 minutes.
Transfer into the clean bowl and cover with a plastic wrap, so it touches the surface of the custard. Cool completely, about 1 hour.
While custard is cooling, slice the strawberries. Pour the cooled custard into the Oreo crust and spread it evenly. Arrange the strawberry slices on top. Refrigerate till ready to serve.
Dust some powdered sugar right before serving, if desired.