Preheat oven to 400 °F. Line an 8" springform pan with 2 layers of parchment paper. Parchment should cover the entire bottom and sides of the pan with 2-3" collar above the pan. Press in as tightly as possible around the bottom and edges.
Chop the chocolate in peas size chunks. Heat the whipping cream in a large microwaveable container. Divide the cream in half and add the chocolate to one half. Leave the mixture for the chocolate to melt. (See Note 1 if making a fully chocolate cheesecake)
Mix cream cheese, sugar and salt in a large bowl. Beat on medium until fluffy. Add eggs one by one and beat until evenly mixed. Add vanilla and mix well. (See Note 1 if making a fully chocolate cheesecake)
Mix the flour in with the mixer on low.
Divide the cream cheese mixture roughly in half in to 2 separate bowls. (I scraped mine into a large measuring cup so I could see when I had poured out half.)
Mix the plain half of the whipping cream in to one half of the cheese mixture in its bowl.
Stir the remaining cream and melted chocolate until well and evenly mixed. Mixture should be completely smooth. Add the chocolate mixture to the other half of the cheese mixture. Add the cocoa powder and beat until completely mixed.
Pour the chocolate batter into the spring form pan and tap the pan on the counter to level it. Carefully pour the plain cheese mixture into the center of the chocolate mixture. Do not stir. Pour slowly and evenly and the edges should remain pretty much chocolate and the center will be more marbled chocolate and vanilla.
Bake 40 -45 minutes. The top will be dark, burnt looking brown and the center will be still slightly jiggly.
All cheee cake to come to room temperature. If you serve it same day (without refrigerating) the center will be creamy and a bit slurry.Alternatively you can refrigerate until ready to serve and the texture will be firmer. When ready to serve carefully release the spring form ring and gently pull the parchment away from the cheesecake. Gently transfer the cheesecake to a serving plate. Cut in wedges and serve.
Cheese cake will keep nicely a few days in the fridge. It can also be frozen, sealed and thawed in the fridge when ready to serve.