How to get those professional looking peaks on top of tiramisu?
You probably wont be able to get that exact result with a pastry bag because the almost perfect square grid they are divided into. I still think a pastry bag will be your best bet.

I am not sure were you got that picture but my guess is that the tiramisu was machine made and the marks and peaks are from the tooling that was made for the machine to deliver the mascarpone topping.

I would try a good sized round tip on a pastry bag and make my dot and slowly raise up and cut it off by stopping the pressure on the bag and pulling straight up.  It has worked for me in the past.

My background is not in pastry though so you may also just go to a local bakery or wedding cake designer and ask how they would do it.  They will probably tell you.