Preheat oven to 160ºC Butter inside of springform pan well & cover the outside (bottom & sides) with a sheet of heavy-duty aluminum foil, shiny side out. This reflects heat away from cheesecake & prevents it from baking too fast & becoming overcooked. Mix graham cracker crumbs with 2 tb of the sugar & add melted butter. Press evenly on bottom & sides of pan; refrigerate until ready to use. Cut chocolate into small pieces. Melt in microwave; combine with rum. Set aside. Beat cream cheese with electric mixer until ight & fluffy. Gradually beat in sugar, sour cream & vanilla. Add eggs, one at a time. Mix well. Place bowl over a pan of hot water & mix until smooth (do not let water touch bottom of bowl). Pour about 1.25 cups of this batter into a separte bowl & set aside. Whisk remaining batter w/the chocolate, then stir over hot water until smooth. Take springform pan from refrigerator & fill w/chocolate batter. Gently pour plain batter over top & make swirls down into the chocolate batter with a fork. Place on middle rack of oven & bake for 50 minutes. Cool to room temp, remove foil & rim of pan & refrigerate overnight.