Rob Kardashian: It's My Birthday ... I DESERVE a Little In-N-Out!

Rob Kardashian unwrapped the best gift of his 29th birthday -- with a little help from Blac Chyna -- late Thursday night ... some In-N-Out!

The couple hit the drive-thru of one of the famed L.A. burger joints around 11 PM for what we hope was a burger and fries. As we've reported, Rob and Chyna have said "no cheat days" on their fitness kick.

But c'mon ... ya gotta indulge on your birthday, right? We're rooting for a double-double ... animal style.

Let the man live!!