Prepare the Pan:

1. Prepare a 9-inch Springform Pan: Line the bottom with parchment paper and lightly grease the sides with butter or non-stick spray. This will help the cheesecake come out cleanly once baked.

Make the Chocolate Layer:

1. Melt the Chocolate: In a heatproof bowl, melt the dark chocolate and milk together over a double boiler or in the microwave (in 20-second intervals, stirring in between). Stir until smooth and let it cool slightly.

Make the Orange Layer:

1. Combine Orange Juice and Zest: In a small bowl, mix the orange juice and zest together. Set aside.

Prepare the Cheesecake Batter:

1. Beat the Cream Cheese: In a large bowl, use a hand mixer or stand mixer to beat the softened cream cheese and sugar until smooth and creamy.
2. Add Sour Cream and Orange Zest: Stir in the sour cream and orange zest, mixing until fully combined.
3. Incorporate Egg Yolks: Add the egg yolks, one at a time, mixing well after each addition. Then, mix in the vanilla extract.
4. Sift in Dry Ingredients: Sift the flour and cocoa powder together and fold them gently into the cream cheese mixture until smooth.

Whip the Egg Whites:

1. Whip the Egg Whites: In a clean, dry bowl, use a hand mixer to beat the egg whites with cream of tartar until soft peaks form. Gradually add sugar (1 tbsp at a time) and continue beating until stiff peaks form. Be careful not to overbeat.

Fold the Mixtures Together:

1. Combine Egg Whites with Cream Cheese Mixture: Gently fold the whipped egg whites into the cream cheese mixture, a little at a time, until just combined. Be careful not to deflate the batter.
2. Divide the Batter: Split the batter into two portions. Add the melted chocolate to one half and the orange juice mixture to the other half. Gently fold until the chocolate and orange mixtures are smooth.

Layer the Cheesecake:

1. Layer the Batters in the Pan: Spoon alternating dollops of the chocolate and orange batters into the prepared springform pan. Then, use a knife or skewer to gently swirl the two batters together to create a marbled effect.

Bake the Cheesecake:

1. Prepare a Water Bath: Place the springform pan in a larger pan, and pour hot water into the larger pan until it reaches halfway up the sides of the springform pan. This water bath helps the cheesecake cook evenly and prevents cracking.
2. Bake: Preheat your oven to 320°F (160°C). Bake the cheesecake for about 50-60 minutes, or until the top is lightly golden and a toothpick inserted into the center comes out mostly clean (it will still be a little moist).
3. Cool in the Oven: Turn off the oven and leave the cheesecake inside with the door slightly ajar for 1 hour to cool gradually. This helps prevent cracking.
4. Chill: After an hour, remove the cheesecake from the oven and water bath. Let it cool to room temperature, then refrigerate for at least 4 hours, or overnight for best results.

Serve and Enjoy:

• Unmold and Slice: Carefully remove the cheesecake from the springform pan and transfer it to a serving platter. Slice and serve chilled.
• Optional Garnish: Garnish with chocolate shavings, orange slices, or a dusting of powdered sugar for an extra special touch.