Preheat oven to 325°F. Grease a 9-inch springform pan.
In a medium bowl, combine the graham cracker crumbs, 1/4 cup sugar and 1/2 cup melted butter. Press mixture into the bottom of the prepared pan.
In a large bowl, beat cream cheese until light and fluffy. Gradually beat in condensed milk until smooth. Beat in eggs one at a time, then stir in vanilla.
Stir in flour until blended. Fold in chocolate chips.
Pour half of the batter into the prepared pan. In a small bowl, combine 1/2 cup melted butter and 1/4 cup sugar; stir until blended. Drop spoonfuls of the butter mixture over the batter in pan.
Pour remaining batter over top; swirl with a knife. Bake for 45 minutes or until center is almost set.
Cool on wire rack for 10 minutes; loosen sides from pan with spatula or knife. Cool one hour longer before removing sides of pan.