In a medium bowl, stir together finely crushed graham crackers and melted butter until crumbs are moistened. Press mixture evenly onto bottom of an 8-inch springform pan (may not completely cover bottom). Cover and chill while preparing filling.
In a small saucepan, sprinkle gelatin over milk; let stand for 5 minutes. Heat and stir over low heat just until gelatin is dissolved. Remove from heat. Cool for 15 minutes.
In a large bowl, beat cream cheese with an electric mixer until smooth. Beat in sour cream, sugar and vanilla until well mixed; gradually beat in gelatin mixture. Divide mixture in half. Gradually stir melted chocolate into half of the mixture.
Spoon half of the chocolate mixture over chilled crust in pan; spread evenly. Carefully spoon half of the white mixture over chocolate mixture in small mounds. Using a narrow, thin-bladed metal spatula or a table knife, swirl chocolate and white mixtures. Top with remaining chocolate mixture, spreading evenly; spoon remaining white mixture over chocolate mixture in small mounds and swirl again. Cover and chill about 6 hours or until set.
To serve, using a small sharp knife, loosen cheesecake from side of springform pan; remove side of pan. Cut cheesecake into wedges. If desired, garnish with chocolate curls.
Tip: If using a sugar substitute, use Splenda® Granular, Equal® Spoonful or packets, or Sweet 'N Low® bulk or packets. Follow package directions to use product amount equivalent to ⅓ cup granulated sugar.
Make-ahead Tip: Prepare as directed through step 4, except cover and chill for up to 24 hours. Serve as directed in step 5.