Preheat oven to 300 degrees. To make the crust, combine graham crackers, melted butter and sugar in a medium bowl. Mix well with a fork to combine. Press the mixture into the bottom of a 9-inch springform pan that has been coated with non-stick spray. Set aside.
To make the filling, melt chocolate in a double boiler or medium bowl over simmering water. Set aside to cool slightly.
In a large mixing bowl, combine cream cheese, sour cream, egg, and vanilla. Mix on low speed until smooth and creamy. Fold in sugar and flour. Pour filling into prepared pan.
Drizzle melted chocolate over surface of cheesecake. Using a toothpick swirl the chocolate into the filling, just until decorative swirls form (don't over-swirl, or you'll lose the effect).
Bake 1 hour, or until center is set. Turn off oven, prop open the oven door (not all the way, just slightly), and allow the cake to cool 15 minutes. Transfer pan to a wire rack and cool completely. Cover with plastic wrap and refrigerate until ready to serve.