Prepare the Crust: Preheat your oven to 325°F (163°C). Mix graham cracker crumbs, sugar, and melted butter in a bowl until well combined. Press the mixture into the bottom of a 9-inch springform pan. Bake for 10 minutes and let it cool.
Make the Cheesecake Batter: In a large mixing bowl, beat the cream cheese until smooth. Gradually add sugar and vanilla, beating until creamy. Add eggs one at a time, mixing well after each addition. Stir in the sour cream until fully incorporated.
Divide the Batter: Separate about 1 cup of the batter into a small bowl. Mix the melted chocolate into this portion until smooth.
Layer and Swirl: Pour the plain cheesecake batter over the cooled crust. Drop spoonfuls of the chocolate batter on top. Use a knife to gently swirl the two batters together to create a marbled effect.
Bake: Place the springform pan in a water bath and bake for 50-60 minutes, or until the center is slightly jiggly but set. Turn off the oven and leave the cheesecake inside with the door ajar for 1 hour.
Cool: Remove the cheesecake from the oven and let it cool to room temperature. Then refrigerate for at least 4 hours, preferably overnight.
Serve: Run a knife around the edge of the pan before releasing the springform. Slice and enjoy!