Manna (μάννᾰ) is one of the offerings described for the Theoi in the Orphic hymns. Although it's unclear what, exactly, the ancient Hellens used for it, we can make an educated guess. Manna is sometimes called  'livanomanna'. Livanos is frankincense, 'manna' means 'powdered'. Manna libanou, could be powdered frankincense, and that is my working theory for manna.

I have made a short video on making manna (hint: it's not so difficult, it just takes some grinding) but it will end in fireworks, so there is that! Also, I managed to get the camera to work almost exactly like I wanted it to!

As for the Theoi that were offered manna: Apollon; Artemis; Astrapaios Zeus; Eos;  Erinyes (+ storax); Helios; Hephaistos; Hygeia; Liknites Dionysos; Nike; Palaimon; Silenos, Satyros, and the Bacchai; Thalassa and Thanathos.