Serves 10 to 12
This cake is tall and light and moist with a swirl of dark chocolate.
Preheat the oven to 325 degrees F. In the top of a double boiler, melt the chocolate with 1/4 cup of the sugar, 1/4 cup of the water, and the baking soda.
In the large bowl of an electric mixer, whisk together the flour with the remaining 1-1/2 cups sugar, and the baking powder. Make a well in the center and add the vegetable oil, egg yolks, remaining 3/4 cup water, and vanilla. Beat together until smooth.
Mix 1-1/2 cups of batter with the chocolate mixture until well combined.
In a separate, clean dry bowl of an electric mixer, beat the egg whites with the cream of tartar until they hold stiff, but not dry, peaks. Fold the egg whites into the white batter mixture.
Pour the white batter into the pan. Add the chocolate mixture on top of the white mixture. Use a knife to swirl the chocolate partially into the white batter.
Bake for 1 hour to 1 hour and 10 minutes, or until a wooden skewer or toothpick inserted into the cake comes out free of batter.
Cool the cake upside down, resting on the “feet” of the pan (if the pan has feet) or inverted over the neck of a long necked bottle.
When the cake is completely cool, turn the pan right side up. Slide a metal spatula or knife around the sides of the pan and the tube to detach the cake. Lift the tube to remove the cake. Detach the cake from the pan bottom and transfer it to a serving platter.