I suggest freezing it (I've frozen tiramisu before...), printing out a copy of the Brittney "ice is a solid" post from the TSA Blog, getting to the airport early and escalating to supervisors if necessary. I'm sure they'll try to say you can't take it (too yummy to fly), but if you make a stink and have the rules on your side, you have a fighting chance.

If all escalation and insistence proves useless, do not, under any circumstance, leave it with the TSOs. Better to take it back and give it as a gift to the cleaning or airline staff than to feed the guys who believe tiramisu is a threat to aviation...