Grease the springform pan with butter and set aside.
Using a hand blender or food processor, blend the cream cheese, sugar, and flour for about 15 seconds.
Add the eggs, sour cream, and vanilla and blend until smooth, about 10 seconds more.
Do not overwork the batter.
Blend 1 cup of batter into the melted chocolate. Pour all of the remaining batter into the prepared pan.
Gently drop tablespoons full of the chocolate batter into the cheesecake in evenly spaced circles. Firmly swirl the flat side of a knife through the batter in 8 to 10 figure-eight motions to create a marbled effect.
Set the pan in the middle of a large sheet of heavy-duty aluminum foil about 2 feet long. Bring the two longer ends up to meet, and fold over to seal, creating a "tent" to allow some space on top for the cheesecake to puff up. Bring the short ends of the foil together and press them tightly against the sides of the pan.
Set a trivet or steaming rack on the bottom of the cooker. Lower the cheesecake into the cooker. Pour in enough water to reach halfway up the sides of the spring form pan.
Lock the lid in place. Over high heat, bring to high pressure. Reduce the heat just enough to maintain high pressure and cook for 35 minutes. Turn off the heat and allow the pressure to drop naturally, about 15 minutes. Remove the lid, tilting it away from you to allow excess steam to escape.
Let the cheesecake cool for a few minutes before removing it from the cooker. Set it on a cooling rack, remove the foil, and let it cool slightly. Serve warm with a spoon or cool to room temperature, cover, and refrigerate at least 6 hours or overnight. Before serving, let the cheesecake come to room temperature, remove the spring form pan, and cut into slices.
This recipe yields 8 servings.
I like the texture best when it's served freshly prepared and slightly warm, even though you can't cut it into tidy slices. It gets firmer and denser—like a New York-style cheesecake—after overnight refrigeration. For optimum flavor, bring it back to room temperature before serving.
The cheesecake freezes beautifully and can be popped straight from the freezer into the microwave for a quick defrost—about 20 seconds on high for a slice.
You'll need a 7-inch spring form pan to prepare this recipe. For a few more pointers, check the Tips below.
Tips: Don't substitute reduced-fat products in this recipe as they contain more water and the cheesecake won't set properly.
Bring the cream cheese to room temperature so that it will easily blend into the other ingredients.
Use a fine-eating bittersweet chocolate bar, not baking chocolate.
If there's a tiny puddle of water in the middle when you unwrap the cheesecake, gently sop it up with a paper towel.
• Lemon Cheesecake: Omit the vanilla extract and chocolate, and blend the zest of 2 lemons (about 1 tablespoon) into the entire batch of batter. Garnish the whole cheesecake or individual slices with raspberries, blueberries, or sliced strawberries, if you wish.