Preheat the oven to 350 degrees F, and grease and flour a 10-cup bundt pan.
In a large mixing bowl, cream the butter, cream cheese, and sugar together on medium-high speed, until very pale and fluffy (about 5 minutes).
Add the eggs, one at a time, stopping to scrape the bottom and sides of the bowl with a silicone spatula after each addition.
Add one cup of flour at a time, stirring on low speed until incorporated.
Stir in the salt and vanilla.
In a smaller bowl, whisk the cocoa and water together.
Fold about half the cake batter into the cocoa mixture.
Drop dollops of chocolate batter into the prepared pan, alternating with vanilla batter.
Swirl together with a toothpick or bamboo skewer.
Repeat, layering on dollops of batter and swirling, until all the batter has been added.
Bake for 70 to 80 minutes, or until a toothpick inserted in the thickest part of the cake comes out clean or with a few moist crumbs.
Cool for 30 minutes, then invert onto a serving plate and dust with powdered sugar.