Tip:To slice the marble cake neatly, use a hot knife (run it under hot running water and dry it). Wipe the blade clean between slices.
Position an oven rack in the middle of the oven and heat the oven to 300°F. Lightly grease a 9x2-inch round cake pan and line the bottom with parchment.
Make the vanilla batter: In a medium bowl, beat the softened cream cheese with an electric mixer until smooth. Add the sugar and continue beating until well blended and no lumps remain. Add the egg and vanilla and beat just until blended. Set aside.
Make the chocolate batter: In a medium bowl, melt the chocolate and butter in a large metal bowl over a pan of simmering water or in the microwave. Whisk until smooth and set aside to cool slightly. With a stand mixer fitted with the whip attachment (or with a hand mixer), beat the eggs, sugar, rum or espresso, vanilla, and salt on medium high until the mixture is pale and thick, 3 to 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 inches from the edge comes out gooey but not liquid, 40 to 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 hours. Loosen the 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 the 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 hours or overnight, or freeze.
Sprinkle cocoa on the bottom of the cake before inverting it onto another plate; the cocoa will keep the cake from sticking when you slice and serve it.
Wrap the cooled cake (unmolded as directed in the recipe) in plastic and refrigerate until firm and well chilled. Slide the cake from the plate and wrap it again in plastic. Freeze for up to a month. To serve, unwrap the cake and set it on a flat serving plate. Cover with plastic wrap and thaw in the refrigerator overnight, or at room temperature for an hour or two.