It is an exclamation, and you are allowed to drop the /essere/ verb, which (in these cases) would just be linking the subject with its qualifier or qualifers, therefore not adding any meaning. The exclamative pattern: (p1) A questo S ! where A is an adjective (or adjectival locution) and S is the subject of the sentence. is fully expandend in the affirmative pattern: (p2) questo S e` A. As you can see /e` / is just a link between S and A, it does not carry meaning. Obviously there are variations in the plural, in the tenses, etc.

"Buona questa pasta al pesto" literally is "Good this pasta with (very literally "at") pesto". You're right, the verb "essere" is not there. In this sentence, the verb is "sottinteso", meaning it's implied in the sentence. This is a form of conversational Italian, I'd say. It's hard to explain, but somehow the sentence works even if the verb is not explicit. You can do it pretty much with anything. Examples: "Bravo questo ragazzo" (This is a good boy) "Bello questo vestito" (This is a pretty/beautiful dress) The sentence, "Questa pasta al pesto è buona" is correct and you can use it, it's just a matter of preference in this case. Hope this helps! If you need any more clarification, feel free to ask.

The full sentence should be "è buona questa pasta al pesto" Here we have an inversion between subject, verb and adjective, the standard form is (as you wrote): "questa pasta al pesto è buona" The inversion is used to emphasize the sentence (I think this happens in Japanese language too) Dropping the verb "essere" at the beginning of the sentence, it changes to be more like an exclamation. For example: "Sei un cane cattivo!" (the owner scold his/her dog "you are a bad dog") "Cattivo cane!" (sei un cattivo cane)