Goes to Town on Birthday Cake,

Miley Cyrus celebrated her boyfriend Liam's birthday last weekend by licking a birthday cake that would make most men feel ... just plain inadequate.

The massive phallic confection was rolled out Saturday night at Club Icon in Downtown L.A. -- and 19-year-old Miley quickly jumped at the photo op ... posing alongside the ... well, just click to see for yourself.

Liam probably wasn't into blowing out the candles so we're guessing Miley did the job for him.

Some party goers tell us they think 19-year-old Miley was drinking alcohol and partying all night.