Pelanoi were Athenian sacrifical cakes that were often used in the Lesser mysteries and in sacrifices to Zeus. As a true Athenian, local, cake, they were also staples in sacrifices to Athena.

It's described that pelanoi were made from aparche (ἀπαρχὴ), 'first-fruit'.  This was the first and most pristine portion of the harvest. The pelanoi used at the Mysteries were made from wheat flour obtained from the plain of the Rharus, a plain of Attica where corn was first sown by Triptolemos, who is a well known figure in the mythology surrounding the Mysteries.

Mythologically, the foundations of the Eleusinian mysteries can be found in the Homeric Hymn to Demeter. Within the hymn, Demeter travels the globe in her grief over losing Her daughter. She eventually settles at the home of Keleus (Κελεός), husband of Metaneira, father of several children, who are called Kallidice, Demo, Kleisidice, Kallithoe, Triptolemos, and Demophon, his youngest son by Metaneira. The daughters of Keleus find a disguised Demeter near a well and bring Her home.

Keleus hires Her to take care of Demophon. He treats her well, with every courtesy, and as a gift to Keleus, because of his hospitality, Demeter plans to make Demophon immortal by burning his mortal spirit away in the family hearth every night. Before she can complete Her work, Metaneira interrupts Her and pulls Demophon from the fire prematurely. This ruins any chance Demophon would have had at immortality. Demeter, after She could no longer take care of Demophon, nor save him from his own mortality, instead taught Triptolemos the secrets of agriculture--a valuable gift, because the art was unknown to mankind until then.

So how does one make a pelanos? Well, we'll never know for sure, I fear, but it seems it was--like all ancient Hellenic 'cakes'--more of a flatbread than a cake as we know it. Even more: it was most likely a cake that was not baked or otherwise subjected to heat. It was, as it is described, a mixture offered to the Gods, of meal, honey, and oil.

Jane Ellen Harrison in 'Prolegomena to the Study of Greek Religion' describes its as follows:

"You first sprinkle the [wheat] meal on the water, you then stir it, so far you have porridge; if you let it get thicker you must knead it and then you have oat-cake. It has of course frequently been noted that a pelanos may be either fluid or solid, and herein lies the explanation. When the pelanos is thick and subjected to fire, baked, it becomes a pemma, an ordinary cake."
How much water and how much meal was unclear, but I would say the measurements used in the recipe for general honey cakes would suffice as these become solid flat cakes very easily:

- 200 gram flour
- 100 ml water
- 3 tbsp clear honey
- 2 tbsp olive oil

During the ritual, the pelanos is supposed to be sacrificed with the left hand, like all offering to the Khthonic Gods, on an eschára (ἐσχάρα), a low-lying altar to the khthonic deities. This was thus, in general, not a sacrifical pit and it would have held a fire. Like all offerings to the Kthonic deities, the pelanos is given as a holókaustos and is thus given in its entirety. You are not supposed to eat of it. Happy sacrificing!