Preheat your oven to 325°F (160°C). Grease a 9-inch springform pan with cooking spray or butter.
In a medium bowl, combine the graham cracker crumbs and melted butter until well blended. Press the mixture firmly into the bottom of the prepared springform pan to form an even crust. Bake for 10 minutes, then let cool slightly.
In a large mixing bowl, beat the cream cheese with an electric mixer until smooth. Gradually add in the sugar, beating until fluffy.
Add the vanilla extract and mix in the eggs one at a time, mixing on low just until incorporated. Be careful not to overmix.
Fold in the sour cream until the mixture is creamy and smooth.
In a microwave-safe bowl, melt the chocolate chips in 30-second increments until fully melted and smooth. Stir in the cocoa powder and a pinch of salt.
Divide the cheesecake batter in half. Take one half and mix it with the melted chocolate to create the chocolate batter.
Alternately pour the plain and chocolate cheesecake batters into the springform pan, starting with a layer of the plain batter. Use a knife or skewer to gently swirl the two batters together, creating a marbled effect.
Bake in the preheated oven for 50-60 minutes, or until the center is set and slightly jiggly.
Turn off the oven and leave the cheesecake in the oven for an additional hour to cool gradually.
Once cooled, remove from the oven and let it reach room temperature before refrigerating for at least 4 hours or overnight to set completely.
Carefully run a knife around the edge of the springform pan before releasing the sides.
Notes
Serve chilled, garnished with whipped cream and chocolate shavings or fresh berries for added color and contrast.