Preheat oven to 325F (160C) and line a 9×9" baking pan with parchment paper or foil.

Place your Oreo cookies in a food processor and process until pulverized to fine crumbs.

Add melted butter and pulse again until well-combined.

Pour cookie crumb mixture into prepared baking pan and use the back of a spoon to press the crumbs firmly into an even layer. Set aside.

Chop your chocolate bars into small pieces (if using chocolate chips, you do not need to chop) and place in a medium-sized bowl.  Microwave for 30 seconds, stir well.  Microwave again for 15 seconds and stir again, repeat until chocolate is smooth and completely melted.

In a stand mixer (or in a large bowl with a hand-mixer), beat together softened cream cheese and sugar, scraping down the sides with a spatula occasionally to ensure ingredients are mixed well.

Add sour cream and vanilla extract and stir well, pausing once to scrape down the sides and bottom of the bowl.

With mixer on low-speed, add melted chocolate and stir until completely combined.

Add eggs, one at a time, stirring on low-speed until just combined after each addition. Scrape down the sides and bottom of the bowl once more to ensure everything is well-combined.

Pour chocolate cheesecake batter evenly over prepared cookie crust.  Bake on 325F (160) for 40 minutes or until cheesecake is just set and slightly jiggly to the touch.

Allow cheesecake to cool to room temperature, and then transfer to refrigerator and chill at least 6 hours before slicing and serving.

Top with whipped cream, if desired.