I believe it's the front cover of some sort of retrospective book, the title of which translates as "Fifty Years of Very Happy Work", a title which the drawing makes clear is dripping with irony!
It is, sort of; Cinquante ans de travaux fort gais
was written by Hergé, and prepared by Casterman for presentation to guests at a cocktail party held to celebrate his golden jubilee as author of Tintin. Rather than being a retrospective, per se, he talks about his process for drawing an album.
To do this, he uses an otherwise un-published page of Picaros (which interestingly doesn't
feature Tintin), and shows the steps from initial rough sketches, through line art, to a finished complete page, by way of a demonstration of a page of colouring, where an acetate sheet allows the reader to lay the black line art over a page of flat colours.
It's a reasonably slim volume, made to the same dimensions as a standard album; the contents were also included within the Rombaldi collection a few years later (possibly without the overlay acetate?).
The title is indeed highly ironic, and given Hergé has drawn himself in prison garb with a ball-and-chain, I like to think of it being closer to Fifty Years of Jolly Hard Labour
He did do more than one version of the theme: the cover of this book for example, by Phillippe Goddin, Comment nait une bande dessinée
, may be the one you are thinking of.