Her Majesty celebrated her "official" birthday Saturday -- which is actually different than her real DOB, April 21 -- but the day prior, Liz cut a ceremonial slice of a cake that had been trotted out to her at the Eden Project in Cornwall ... where she was on hand for G7 duties, among other world leaders -- including Joe Biden, Boris Johnson and others.

Camilla Bowles was actually there too, as was Kate Middleton ... who watched QE2 attempt to cut her bday cake with a super long blade which we guess is ... tradition??? Who the hell knows -- point is, the Queen seemed dead-set on getting it done, despite her vocal hesitations.

At one point, someone on her staff made a quick note as HRH seemed to struggle a bit with the awkward feat -- noting there was a knife there too, if that was easier for her.

Watch what happens ... Elizabeth shot the aide a WTF look, and told her straight ... "I know there is." Her quick wit and sharp tongue got a huge chuckle from everyone around her. No harm, no foul obviously -- but we must say ... her putting the aide in her place is hilarious.

BTW, the jokes from the Queen didn't end there ... she got even more laughs that same day when she was posing on stage with the shot-callers of the free world -- openly asking if they were just supposed to sit there and seem like they were enjoying themselves. Great delivery!

Oh, and we should also mention ... QE2 got a proper celebration at Windsor to kick off her weekend on Saturday itself -- with a giant procession that was full of pomp and circumstance ... and rightly so. She's a ripe 95 years old now -- and still going strong even at this age.