Beat butter at medium speed of an electric mixer until creamy. Gradually add 3 cups sugar, beating at medium speed 5 to 7 minutes. Add 5 eggs, one at a time, beating just until yellow disappears.
Combine flour and next 3 ingredients; add to butter mixture alternately with milk, beginning and ending with flour mixture. Mix at low speed just until blended after each addition. Stir in melted chocolate and vanilla. Set batter aside.
Beat cream cheese at medium speed until smooth. Add 1/4 cup sugar, beating well. Add 1 egg and cornstarch, beating until smooth. Pour half of chocolate batter into a greased and floured 10" tube pan; spoon cream cheese batter over chocolate batter, leaving a border around outer edge. Spoon remaining chocolate batter over cream cheese layer.
Bake at 325° for 1 hour and 40 minutes or until a wooden pick inserted in center comes out almost clean. (Do not overbake.) Cool in pan on a wire rack 30 minutes; remove from pan and let cool completely inverted on wire rack. Pour Fudge Frosting over top of cake.