1. Grease 7-inch springform pan with butter and set aside.
2. Using food processor, blend together cream cheese, sugar and flour for about 15 seconds. Add eggs, sour cream and vanilla; blend until smooth, about 10 seconds more. Do not overwork batter.
3. Blend 1 cup of batter into melted chocolate. Pour remaining batter into prepared pan. Gently drop tablespoonfuls of chocolate batter into cheesecake in evenly spaced circles. Firmly swirl flat side of a knife through batter in 8 to 10 figure-eight motions for a marbled effect.
4. Set pan in middle of large sheet of heavy-duty aluminum foil, about 2 feet long. Bring two longer ends up to meet; fold over to seal, creating a tent and allowing some space on top for cheesecake to puff up. Bring short ends of foil together on both sides and press them tightly against sides of pan.
5. Set a trivet or steaming rack on bottom of cooker. Lower cheesecake into the cooker. Pour in enough water to reach halfway up the sides of the springform pan.
6. Lock lid in place on cooker. Over high heat, bring to high pressure. Reduce heat just enough to maintain high pressure; cook for 35 minutes.
7. Turn off heat and allow pressure to drop naturally, about 15 minutes. Remove lid, opening it away from you, letting any excess steam escape.
8. Let cheesecake cool a few minutes before removing from cooker. Set on wire rack. Remove foil and let cake cool slightly. Serve warm with a spoon, or cool to room temperature, cover and refrigerate at least 6 hours or overnight. Before serving, let cheesecake come to room temperature. Remove sides of pan. Cut cake into slices. Makes 8 servings.