Preheat oven to 325 degrees F (165 degrees C). In a medium bowl, combine graham cracker crumbs and butter. Mix well and press into the bottom of a 9 inch springform pan. Bake in preheated oven for 10 minutes. In a microwave-safe bowl, microwave chocolate until melted. Stir occasionally until chocolate is smooth.
In a large bowl, beat the softened cream cheese, sugar and vanilla until creamy and smooth. Blend in the eggs one at a time. Remove 1 cup of cream cheese mixture and combine with melted chocolate.
Spread plain batter into the crust. Drop the chocolate mixture randomly over the top. With the tip of a knife, Swirl the batter to create a marble effect.
Bake in the preheated oven for 30 to 35 minutes, or until center is set. Cool to room temperature, then refrigerate for 3 hours or overnight before serving.
