Tom Cruise -- Does Ex Factor ... With Penelope Cruz

Tom Cruise hooked up with his ex-girlfriend Penelope Cruz ... for dinner in London and left with a souvenir... -- he was caught leaving with a red lipstick stain on his cheek.

Since he's Tom Cruise ... dinner with Penelope wouldn't be hot enough -- so her pal Salma Hayek came along for the meal at Scott's in London.

No clue on who planted the smooch on Cruise -- both women were wearing red lipstick.

Now, for the buzz kill -- Penelope's married with kids now (just like Salma) so probably not a hot and heavy date night.

Ahh, but the mammaries. They'll always have those.