I chose a sable dough, though you could totally use whatever cookie recipe holds up well for you (gingerbread would be cheery!).
Since the whole message/envelope shtick seems to lend itself to mailing the cookie, you probably just want to make sure you’re picking a recipe sturdy enough to stand up to some jostling and dropping, well-packed as it would be.
The same royal icing/glue recipe I use for gingerbread was pressed into service for this project as well. I have fallen into the habit of using the dried egg whites just to anticipate people who may be sensitive to regular, raw egg whites, and for the ease of having a shelf-stable material on-hand.
Not sure about the opening mechanism for the “envelope”, I opted to have a thick strip of cookie that could be snapped off, much like the “fold and tear” perforations on some mailings. Otherwise, I went with the best cookie proportions that seemed to work for my design (especially taking into account the size of the cookie cutter I was using for the interior message and the minimum thickness I would need for that message).
It was actually a pretty low-fuss process, and it would totally lend itself to all sorts of different shapes and levels of fanciness. Maybe next time I’ll use them as invites to a tea party or something.