Preheat your oven to 325°F (160°C) and generously grease a 9-inch springform pan with cooking spray or butter to prevent sticking.
In a medium mixing bowl, combine the graham cracker crumbs with the melted butter. Stir until the crumbs are evenly coated. Press this mixture firmly and evenly into the bottom of your prepared springform pan to form a solid crust. Bake for 10 minutes, then allow it to cool for a few minutes.
In a large mixing bowl, use an electric mixer to beat the softened cream cheese on medium speed until it is completely smooth and creamy. Gradually add the granulated sugar, continuing to beat until the mixture is light and fluffy.
Add the pure vanilla extract, then introduce the eggs one at a time, mixing on low speed until each egg is fully incorporated before adding the next. Take care not to overmix the batter.
Gently fold in the sour cream until everything is well combined and the mixture is smooth.
In a microwave-safe bowl, melt the semi-sweet chocolate chips in 30-second intervals, stirring in between until they are fully melted and smooth. Mix in the unsweetened cocoa powder and a pinch of salt until fully blended.
Divide your cheesecake batter in half. Take one half and carefully mix it with the melted chocolate to create a rich chocolate batter.
Pour the plain cheesecake batter into the springform pan first, followed by the chocolate batter. Alternate the remaining batters in the pan, and use a knife or skewer to gently swirl them together to achieve a beautiful marbled effect.
Place the springform pan in the preheated oven and bake for 50-60 minutes or until the center of the cheesecake is set but still has a slight jiggle.
Turn off the oven and leave the cheesecake inside for an additional hour to cool gradually, which helps prevent cracking.
Once cooled, remove it from the oven and allow it to come to room temperature. Then, refrigerate for at least 4 hours, or ideally overnight, to ensure it sets completely before serving.
Before removing the sides of the springform pan, carefully run a knife around the edge to loosen the cheesecake. Then, gently release the sides of the springform pan.
Notes
For best results, refrigerate overnight before serving.