Beat egg whites until soft peaks form. Gradually add 1/2 cup sugar, beating until stiff peaks form; set aside.
In a large bowl, combine the flour, baking soda, salt and remaining sugar. Add oil and 1/2 cup buttermilk; beat 1 minute. Add the yolks, chocolate and remaining buttermilk; beat 1 minute. Fold in egg white mixture. Pour into two greased 9-in. round baking pans.
Bake at 350° for 30-35 minutes or until a toothpick inserted in the center comes out clean. Cool for 10 minutes before removing from pans to wire racks to cool completely.
For frosting, cream butter and cream cheese. Add confectioners' sugar alternately with milk. Gradually add chocolate and vanilla; mix well. Frost cake. Cover and chill.