Heehee, my hubby and children are fluently bilingual also. I am not totally fluent but can get by. Actually my hubby has been involved in French Language services for much of his career. He retired from the Ontario government 8 years ago and has since been Directeur General of an organization that includes all of the French speaking municipalities of the province of Ontario. My oldest daughter recruits for her Alma Mater, the local bilingual university. Heehee, so your hubby could get along with mine!
Perhaps in the U.S. culinary and patisserie books, this term is used for something different, but a gateau is a cake in French, Parisian or Canadian, doesn't matter. The language is exactly the same, just some expressions are more common in one place or the other. The plural term is "gateaux". Here are two definitions from other culinary sites for gateau.
GATEAU: This is the french word for cake. It is traditionally a multi-layered cake filled with cream fillings or buttercreams, rectangular or round and more decorative than the American style layer cake.
Gateau (French) Cake
Still no luck on finding this term for cake stencilling with powdered sugar.
"Champ" is the noun for field. Is it possible the term is something like, "champ de blanc" which translates to field of white? Or could the term be, "champ du blanc"? Or is it possible that the word is "champetre" which is the adjective for rural, as this was kind of a country way of decorating back many years?
Other variations of "blanc", the feminine, "blanche", "blanchir" , to whitewash or to refine when used in connection with sugar.