Marble Chiffon Cake

Directions

1. Let eggs stand, separated, at room temperature for 30 minutes. In a small saucepan, melt unsweetened chocolate over low heat. Add 1/4 cup sugar and hot water; mix well and set aside.
2. In a large bowl, combine the flour, baking powder, salt, baking soda and remaining sugar. Whisk together the egg yolks, water, oil and vanilla; add to flour mixture and beat until moistened. Beat for 3 minutes on medium speed; set aside.
3. In another large bowl and with clean beaters, beat egg whites and cream of tartar on high speed until stiff peaks form. Fold a fourth of egg whites into the batter, then fold in remaining whites. Divide batter in half; gradually fold chocolate mixture into 1 portion.
4. Alternately spoon the plain and chocolate batters into an ungreased 10-in. tube pan. Swirl with a knife. Bake on the lowest rack at 325° for 60-65 minutes or until top springs back when lightly touched. Immediately invert cake; cool completely. Run a knife around sides and center tube of pan; remove cake to serving plate.
5. For frosting, melt semisweet chocolate and butter in a small saucepan over low heat. Stir in cream and vanilla. Remove from the heat; whisk in confectioners' sugar until smooth. Immediately spoon over cake.