Heat the oven to 300°.
Lightly grease a 9-inch springform pan and line the bottom with parchment.
To make the vanilla batter, beat the softened cream cheese with a mixer until smooth.
Add the sugar and continue beating until well blended and smooth.
Add the egg and vanilla and beat just until blended. Set aside.
To make the chocolate batter, melt the chocolate and butter in a microwave safe bowl in the microwave.
Whisk until smooth and set aside to cool slightly.
Using a stand mixer with the whisk attachment, beat the eggs, sugar, vanilla, and salt on medium high until the mixture is pale and thick, 3 to 4 min.
With the mixer on low, slowly pour in the chocolate mixture and continue beating until well blended.
Spread about half of the chocolate batter in the bottom of the pan.
Alternately add large scoopfuls of each of the remaining batters to the cake pan.
Using a knife, carefully swirl the two batters together so they're mixed but not completely blended. Tap the pan against on your counter several times level the batters.
Bake until a toothpick inserted about 2 inches from edges comes out with a few moist crumbs, but no gooey batter, about 40-42 minutes.
Cool on rack about 1-1/2 hours.
Remove sides of springform pan and use spatulas to remove cake to serving plate.
Cover and refrigerate until cold.
To serve, slice with a knife dipped in hot water, then dried.
Yield: 8-10 servings
