Why did my Tiramisu Cake become dry? I followed Dorie Greenspan's recipe http://clclt.com/eatmycharlotte/archives/2010/04/09/dorie-greenspans-tiramisu-cake for making the sponge cake. The cake was very soft when it cooled down.

However, after sitting in the fridge overnight, it was quite tough. It was left uncovered in the fridge. Not sure if this caused it - if yes, what is the best way to keep it in the fridge while the flavours blend?
I would say the refrigeration is the culprit. Fridges suck the moisture out of anything that isn't tightly covered.

I would either wrap the cake in plastic wrap or put in an airtight tub or cake tin before refrigerating.

This will also prevent the cake from absorbing any smells in the fridge: chocolate is very prone to this.