A Tim Tam is a biscuit
, about 2 cm wide, 4 cm long, and 1 cm thick, or so, but don't take me up on that, it wasn't designed by Michelangelo
and it's far too long since I had one, besides.
It's just two pieces of biscuits sandwiched together with chocolate cream, and chocolate all around the outside. (These days they also make variants: probably things like dark chocolate, coffee, mint, that sort of thing that you can do to the chocolate without violating it too much.)
Here is the important bit, to readers in the UK who haven't had a Tim Tam: the above description exactly fits the Penguin biscuit. Here is the difference: the Tim Tam is vastly more delicious (different proportion of cocoa butter? I don't know why) and no adult has ever got addicted to Penguins.
Tim Tams are probably the most delicious, moreish, shit-there-go-my-teeth-and-my-belly-but-who-cares biscuit ever invented, by anyone, ever. National chauvinism doesn't come into it here. All Britons who go there (or are privileged to partake of the food parcels that expatriate Australians in the UK regularly get sent to assuage their cravings), and who begin by observing that they look just like Penguins (except the wrapper is elegant, not vulgar), are converted and agree there is no comparison.
It can produce some odd conversations at Customs at Heathrow, if for some reason they flag you down.
"What's in these suitcases?"
"Chocolate biscuits and...?"