I found the exact Marble Chiffon Cake recipe Gavin and Freelancer make in this video and from the baking stream. It's from The Taste of Home Baking Book. If anyone makes it, let me know how it goes! I haven't tried yet.
Let eggs stand at room temperature for 30 minutes. In a small saucepan or microwave, melt unsweetened chocolate; stir until smooth. Add 1/4 cup sugar and hot water; mix well and set aside.
In a large mixing 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.
In another large mixing bowl and with clean beaters, beat egg whites and cream of tartar on high speed until stiff peaks form. Stir a fourth of egg whites into the batter, then fold in remaining whites. Divide batter in half; gradually fold chocolate mixture into one portion.
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 the cake; cool completely. Run a knife around sides and center tube of pan; remove cake to serving plate.
For frosting, melt semisweet chocolate and butter in a small saucepan or microwave; stir until smooth. Remove from the heat; stir in cream and vanilla. Whisk in confectioners' sugar until smooth. Immediately spoon over cake.