Unlike in german the base word is pasto, this mixture of flour and water from which you make all forms of pastaĵoj. One peace I would call pastaĵero or unuopa pastaĵopeco. Aŭ nudelo, spageto, makarono, ..., but then you have to learn many words, this may come later.
pasto has a broader meaning than just pasto for making pastaĵoj. Also the mixture for making cakes you call pasto, but the baked result kuko, or the small ones, eatable as a whole piece, kekso(j). Also bread is made from (a different) pasto.
PIV is THE big one-language dictionary. In my old offline PIV from 1987 I find only the plural form, "pastaĵoj". Online (http://vortaro.net/#pasta%C4%B5oj) you find "pastaĵo" for one peace, too. So it "permits" ;-) that you may use it for one piece. PIV is sometimes criticized being a bit away from practical usage (I agree). For me pastaĵo is still some material, formed from pasto and dried afterwards. Like faruno (flour) and farunaĵo (something formed from flour, water and other stuff, mostly baked afterwards). I would never use "pastaĵo" for just one piece, but would understand it in a context.
Technically, -aĵ- doesn't have a fixed meaning. In the context of food, however, it does mean something like that.
Did you report it?
(I just reported that "Pasta dishes are delicious. ought to be marked as correct, so hopefully it will be marked as correct "soon", as I've reported this suggestion on other questions and got it accepted.)
As of 2016-Jan-08, Pasta dishes are delicious.
is marked as a correct translation of Pastaĵoj estas bongustaj.
(together with "Another correct solution: Pasta is delicious.").
Got the confirming e-mail this night, and got this sentence-to-translate just now.
bona (bon·a) = good, okay, nice
gusti (gust·i ← gust·o) = to taste
bongusta (bon·gust·a ← gust·o) = tasty, delicious
and, as Sukram1947 wrote:
"is [adjective]" "estas [...]-a" = "[verb]" "[...]-as"
bongusti (bon·gust·i ← gust·i) = to taste good
(with an account over on Lernu.net, you get access to both the dictionary at Lernu!, with multiple languages to and from Esperanto, and the Esperanto-Esperanto lexicon Plena Ilustrita Vortaro de Esperanto (vortaro.net))
What I love about Esperanto is just how easy it is to make words. I was looking at the word here used for pasta (pastaĵoj = dough + made of + plural), and I thought to myself "If the word for dough is 'pasto', then you could say 'paste' when calling something 'doughy'." It's not in lernu's dictionary (i.e. not widely accepted), but it's still valid I'd say.