Preheat oven to 325F (160C) and wrap the outside of a 9" springform pan with foil (this is to prevent any leaks, as springform pans are notorious for leaking).
Prepare Oreo crust by placing cookies in the basin of a food processor and pulsing until pulverized to fine crumbs. Combine cookie crumbs and melted butter and stir until crumbs are moistened. Pour into springform pan and press evenly over the bottom and partially up the sides of the pan.
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. Set aside to cool.
In a stand mixer (or in a large bowl with a hand-mixer), cream together softened cream cheese and sugar until smooth, scraping down the sides with a spatula occasionally to ensure ingredients are mixed well.
Add sour cream and vanilla extract and stir on low-speed until combined.
Check the temperature of your chocolate. It should not feel hot to the touch and should not be so cool that it has re-solidified. If it's near room temperature, turn your mixer to low-speed and drizzle chocolate into the batter until well-combined.
Add eggs, one at a time, beating until just mixed after each addition. Do not over-mix or your cheesecake may crack. Scrape the sides and bottom of the bowl and stir on low-speed again until ingredients are completely combined.
Spread cheesecake batter evenly over prepared cookie crust. Wrap the bottom of the pan in foil and place on a cookie sheet (to insure against leaks, springform pans are notorious for them!) Bake on 325F (160C) for 70-75 minutes or until cheesecake springs back to the touch if lightly touched. Edges should be set but the center should still be slightly jiggly like Jello. Do not over-bake or the texture will not be creamy as it should be.
Allow cheesecake to cool to room temperature, and then transfer to refrigerator and chill at least 6 hours before serving. I always store my cheesecake in the springform pan, I remove the edges to slice it and serve it then return the ring around the edges in the refrigerator. If desired, top with homemade whipped cream before serving!
¹Chocolate: I recommend using baking bars but chocolate chips will work. I like to use 60-75% chocolate, but you could also get away with using semisweet chocolate in a pinch ² Cream cheese. Do not use the spreadable type of cream cheese that is sold in a tub or low-fat cream cheese. Use full-fat, the kind that is sold in bricks (you'll need four 8-oz bricks).
Keep in the refrigerator for up to a week or in the freezer for several months. See "Storing" section in post for more details.