• Preheat Oven & Prep Pan: Preheat oven to 350°F (180°C). Line a 9-inch springform pan with parchment paper and grease the pan.
• Make Oreo Crust: In a medium bowl, combine the 3 cups of Oreo crumbs, melted butter, and salt. Mix with a fork until all crumbs are moistened.
• Press and Bake Crust: Press the mixture firmly into the bottom and at least halfway up the sides of the springform pan. Use a flat-bottomed measuring cup for a tight, even crust. Bake for 10 minutes.
• Re-compact Crust: As soon as the crust comes out of the oven, use the same measuring cup to gently re-compact the crust, as it will have puffed up slightly. Let the crust cool on a wire rack.
• Lower Oven Temp & Prep Water Bath: Reduce the oven temperature to 300°F (150°C). Place a 9×13 inch baking pan on the bottom oven rack and fill it with boiling water to create a steam bath.
• Start Cheesecake Batter: In the bowl of a stand mixer, beat the softened cream cheese on low speed for 1-2 minutes until smooth. Scrape down the bowl.
• Add Sugar/Cornstarch: In a small bowl, whisk the sugar and cornstarch together. Add this to the cream cheese and mix on low speed until fully incorporated.
• Add Eggs & Sour Cream: In a small bowl, lightly whisk the eggs and vanilla. With the mixer on low, drizzle the egg mixture in four batches, mixing until just combined after each. Scrape the bowl, add the room-temperature sour cream, and mix on low until just combined. Do not overmix.
• Create Chocolate Batter: Pour about half of the cheesecake batter into a separate medium bowl. Melt the semi-sweet chocolate (in 30-second microwave bursts, stirring in between) and let it cool for 5 minutes. Fold the cooled, melted chocolate into one bowl of batter until smooth.
• Assemble Cheesecake: Using a large scoop, alternate adding large dollops of the vanilla batter and the chocolate batter into the cooled Oreo crust, piling them on top of each other.
• Swirl the Batter: Take a butter knife or a skewer and plunge it to the bottom of the batter. Make 3-4 parallel “swirls” in one direction (e.g., north to south), and then 3-4 swirls in the other direction (e.g., east to west). Do not over-swirl.

Bake, Cool, and Chill:

• Carefully place the cheesecake on the center oven rack (above the water bath). Bake for 1 hour 45 minutes.
• Turn off the oven, crack open the oven door, and let the cheesecake cool slowly in the oven for one hour.
• Remove from the oven, let cool on the counter, then transfer (uncovered) to the refrigerator. Chill for at least 8 hours or overnight to fully set.