The same as the difference between to-mah-to and to-may-to. Both are names for the (usually) round piece of metal that is going to be hammered or struck into a coin. They derive from different languages, but I don't have any handy references in the computer room to see which ones.
From The Numismatic's Glossary, A Comprehensive Listing Of Pertinent Terminology Planchet, blank The disc of metal or other material on which the dies of the coin, token or medal are impressed; also called blank, disc, flan. In paper money, a small colored disc embedded in the paper used as an anti-counterfeiting device. Flan See Planchet.
I was going to post a recipe, but I did not think that was what he was asking about. . So I went with the numismatic definition instead.
I believe flan and blank mean the same thing. That would be the piece of metal punched from the strip. After the blank or flan has had its edge upset or raised in the upsetting mill it is then ready to be struck into a coin and at that point it is a planchet.
The terms 'flan' & 'planchet' both mean the same thing.I use the term 'planchet',as a 'flan' reminds me of the flan rings that a baker has in their kitchen.