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This recipe seems very simple from here.But then for each word you need another recipe and it comes out not being so simple! I tried once. Took half the afternoon but I was happy of the result. This book is the big book from the French chefs of the fifties. Huge book with all the known chefs of the time. Of course cooking has been evoluting since then, but you find all the basics here. I got it from my grand father. The edition is 1957, 1042 pages.
According to the "Dictionnaire de la langue française d’Émile Littré" (one of the greatest dictionary of French languages from the XIXth century) in 1864; the name comes from the fact that you can eat it very quickly, as fast as a lightening (in French éclair means lightening or flash, and in figurative it means something going on very quickly). Another possible explanation is about the bright topping, compared to the brightness of a lightening.