Many years ago, there was a 'confection' called a 'Munchmallow'. These had a biscuit base, topped with a dome of gooey cream like stuff, the entire thing then being covered in chocolate. Apparently either BEA or BOAC used to serve these as a 'snack'. It was found however that in some cases, even in pressurised aircraft, the internal pressure would cause the filling to expand and thus the chocolate to burst, exposing an unsightly 'ring' of white when it was unwrapped.
Research was carried out in the high altitude chamber (decompression chamber) at IAM Farnborough; this resulted in the makers developing a porous biscuit which allowed pressure to equalise inside and outside the chocolate thus keepimg the whole thing intact!