The 'Glee' star described the actor, who died in July last year of a drug overdose, as having ''the biggest heart and most beautiful smile'' and shared a photograph of the pair together.
The black and white image, posted on Lea's Twitter page, shows the couple sharing a joke, and she accompanied it with the caption: ''The biggest heart and most beautiful smile.. In all of our hearts.. We love you so. Happy Birthday. (sic)''
Fans showed their support for Lea and claimed they too were still ''heartbroken'' over the actor's death.
One wrote: ''This photo broke my heart. i miss him so much. Stay strong love i love you. Happy birthday angel Cory. (sic)''
Cory, 31, who starred in 'Glee' with Lea, was found dead in his Vancouver hotel room just months after checking himself into rehab for substance abuse.
A friend close to the star claimed he was ''healthy and in great spirits'' just two days before he passed away.
Lea, 27, previously spoke about her boyfriend's death at the Teen Choice Awards.
She said: ''He was very special to me and also to the world. And we were lucky to witness his incredible talent, handsome smile and his beautiful, beautiful heart.
''So whether you knew him personally or just as Finn Hudson [in 'Glee'], Cory reached out and became a part of all of our hearts - and that's where he'll stay forever.''
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