Rob Gronkowski -- Smashes Football Cake On Bro's Head ... At Epic Vegas Party!!

The Rob Gronkowski party trains rolls on ... there were hot chicks (a lot of them), bottles of champagne, a Lombardi Trophy shaped cake ... and TMZ Sports has the EPIC party video.

The Patriots superstar hit up Surrender Nightclub at the Encore hotel in Vegas last night ... and did what Gronk does -- got super turnt up and smashed stuff!!!

Sources at the club tell us Gronk arrived around 1AM ... pounded champagne straight from the bottle, danced on stage, and showered the crowded with bubbly.

Best part ... Gronk SPIKING the football cake ... on his brothers head!

Check out the video.