Preheat the oven to 350 degrees. Lightly spray a 9-inch springform pan with cooking spray and set pan aside.
Place the Oreos in the bowl of a food processor and process until they form fine crumbs. With processor running, stream in the melted butter. Process until the crumbs are evenly coated in butter, stopping to scrape the sides of the bowl with a rubber spatula as necessary.
Alternatively, you can place the cookies in a large zip-top bag and crush them with a rolling pin, then transfer to a bowl to mix in the butter.
Press the crumbs into the bottom and partway up the sides of the prepared pan. Freeze the crust for 10 minutes.
Remove crust from the freezer and bake for 10 minutes; cool the baked crust on a wire rack while you make the filling.
Place the chopped chocolate in a small heat-proof bowl and microwave in 30-second intervals, stirring after each interval, until chocolate is melted and smooth. Set aside to cool while you proceed.
Place the cream cheese and sugar in a large mixing bowl and beat with an electric mixer on high speed for 5 minutes, stopping the mixer to scrape down the sides and bottom of the bowl as necessary.
Add the eggs, vanilla, espresso powder (if using) and salt and beat to combine. Scrape down the sides and bottom of the bowl, then beat in the chocolate until well combined. Scrape down the sides and bottom of the bowl again, then beat 30 seconds more.
Rap the bowl on the counter a few times to remove any bubbles, then pour the filling into the cooled crust. Bake 35 to 40 minutes, until the edges of the cheesecake are puffed but the center still jiggles a bit with you tap the pan.
Cool cheesecake to room temperature on a wire rack, then cover loosely with plastic wrap and refrigerate for at least 8 hours.
After the cheesecake has chilled, place the chopped chocolate in a small mixing bowl.
Place the heavy cream in a small saucepan set over medium heat and warm the cream until it just begins to steam. Pour the warm cream over the chocolate and let it sit for 3 minutes. Whisk until the chocolate is melted and mixture is smooth.
Pour over the chilled cheesecake and refrigerate for 30 minutes to set the topping. Slice and serve.
For this recipe I used (and recommend) Trader Joe's Pound Plus dark chocolate. Other good options are Ghirardelli, Baker's, and Lindt. You can use chocolate chips in a pinch.
Uneaten cheesecake can be stored, loosely covered, in the fridge for up to 3 days or in the freezer (tightly wrapped) for up to 3 months.
The cheesecake needs to chill overnight in the fridge before serving, so be sure to plan for that.