Made with vegetable oil and kefir, this cake is exceptionally moist and tender. You can find kefir, a cultured milk product, at health food stores and many supermarkets. Be sure to buy the unsweetened, unflavored variety for this recipe. Here the batter is divided in half, with one half left plain and the other mixed with cocoa powder, and the two are layered in the Bundt pan and marbled by running a table knife in a spiral fashion.