The pair of joeys look like they're having a great time with their specially-made "cake" - although we do think they should be wearing their hats to get in the party mood!
Chester and Caeser aren't brothers but they've been best friends since they were born a day apart in April last year.
And they also have three other pals in the three other young koalas who are currently housed at Featherdale Wildlife Park in Sydney, Australia.
We reckon they must get up to all sorts of mischief together!
General curator Chad Staples said: "It's like koala day care centre, so we can keep a close eye on them. The animals are all at the same developmental stage. It makes for a wonderful viewing area seeing five little koalas climbing and exploring and hanging on each other's backs."
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