Ang, I do not recommend subbing the bittersweet for the unsweetened. I tried that once and it was
absolutely inferior, even though I decreased the sugar, etc. I learned (too late) from Alice Medrich's "Bittersweet" that it's not just a matter of decreasing the sugar, but one needs to adjust the chocolate amounts according to the chocolate percentages, etc., plus the fat amounts have to be adjusted, etc.
For example, in Alice Medrich's Bittersweet Chocolate Ice Cream, 3 1/2 oz unsweetened chocolate calls for 3/4 cup sugar - if subbing bittersweet 70%, it calls for 6 oz chocolate, along with 1/2 cup sugar, if subbing bittersweet 60%, it calls for 8 oz chocolate along with 1/4 cup sugar, etc.
**** I strongly urge you to use unsweetened chocolate so you end up with the true chocolate intensity and depth the cake is intended to have!
This recipe is not in T&T. I guess I should post it there.
P.S. I'm glad you found the ladyfingers!