How can I make a coconut cake really coco-nutty tasting?

Coconut manna (coconut butter) has a very strong and lovely coconut flavor. Amazon carries the Nutiva brand which is my favorite. It would make a grat addition to the filling.

use cream of coconut that's used for cocktails in your icing

Some suggestions:

Use a recipe that contains a good amount of coconut milk
Add a good quality coconut extract.
Toast your coconut flakes, if using, to a light golden color
Put coconut into the frosting too.

Joann Chang's Flour cookbook has a terrific coconut cake recipe that I successfully converted to cupcakes.