^Haha, lovely
Unfortunately, a marshmallow the size of planets wouldn't be a normal, fluffy marshmallow right through as gravity would highly condense its core.
Other things I can imagine could be a knitting club where intelligent black holes get together to knit socks and talk about the books written by the Bronte sisters. Or a planet where natural history has made it so that every joke from Gary Larson's The Far Side Comic is accurately reproduced.
edit :
To the question posed in the title : In a universe of infinite space and time [and I would add matter], is it possible for something to happen once?
The simple answer is : Yes, if the probability that the event happens more than once is zero.