Flourless Chocolate & Vanilla Marble Cake

** Recipe from Fine Cooking **

For the vanilla batter:
* 8 oz. cream cheese, softened to room temperature
* 2/3 c. granulated sugar
* 1 large egg
* 1 tsp. pure vanilla extract

For the chocolate batter:
* 10 oz. bittersweet chocolate, finely chopped
* 5 oz. (10 Tbl.) unsalted butter, cut into 6 pieces
* 3 large eggs
* 1/3 c. granulated sugar
* 1 Tbl. dark rum or espresso
* 1 tsp. pure vanilla extract
* pinch table salt

* cocoa powder for dusting

Position an oven rack in the middle of the oven and heat the oven to 300.  Lightly grease a 9x2" round cake pan & line the bottom with parchment.  Make the vanilla batter: In a medium bowl, beat softened cream cheese with electric mixer until smooth. Add sugar & continue beating until well blended & no lumps remain. Add egg & vanilla and beat just until blended. Set aside.
Make the chocolate batter:  In a medium bowl, melt chocolate & butter in a large metal bowl over a pan of simmering water or in the microwave. Whisk until smooth & set aside to cool slightly. With a stand mixer fitted with the whip attachment (or with a hand mixer), beat eggs, sugar, rum or espresso, vanilla, & salt on med-high until the mixture is pale and thick, 3-4 min. With the mixer on low, gradually pour in the chocolate mixture and continue beating until well blended.
Combine and bake: 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 or the tip of a rubber spatula, gently swirl the two batters together so they're mixed but not completely blended. Rap the pan against the countertop several times to settle the batters.  Bake until a pick inserted about 2" from edge comes out gooey but not liquid, 40-42 min.; don't overbake. The top will be puffed and slightly cracked, especially around the edges. It will sink down as it cools. Let cool on a rack until just slightly warm, about 1-1/2 hrs. Loosen cake from the pan by holding the pan almost perpendicular to the counter; tap the pan on the counter while rotating it clockwise. Invert onto a large flat plate or board. Remove the pan and carefully peel off parchment. Sift some cocoa powder over the cake (this will make it easier to remove the slices when serving). Invert again onto a similar plate so that the top side is up. Let cool completely. Cover and refrigerate until very cold, at least 4 hrs. or overnight, or freeze.