The Glee actress uploaded a picture to Twitter on Sunday, 11 May showing herself and Cory giggling together.
Lea wrote: "The biggest heart and most beautiful smile.. In all of our hearts.. We love you so. Happy Birthday."
The brunette's post was favourited more than 100,000 times and retweeted nearly as much.
TV star Cory was just 31 years old when he was found dead in a Vancouver, Canada hotel room in July 2013. It was later determined by the coroner that he died of a combination of heroin and alcohol.
As well as playing Lea's on-screen boyfriend Finn in Glee, he had been the actress' real life partner.
Lea, 27, has spoken out about her grief in interviews since Cory's death. "I literally lived every day of my life feeling like the luckiest girl in the whole world," she said in December 2013. "l thought he was the greatest man."
"[My mum] told me at one point there is an empowerment that comes with grief. At some point you find it. It's hard but you'll find it. I think at a certain point you can choose to fall from this or choose to rise. I'm trying to do my best for him as I know that's what he would have wanted."
In another interview with American ELLE, she said: "I never thought I would be in this position in my whole life. Now that I am in this position, you can choose to rise, and that's what I'm going to try to do.
"I know that Cory would want nothing more than for me to take this situation and use it to help people."
Lea said in January that she feels as if Cory is watching over her now, giving her the strength to grieve and continue with her life.
"I somehow feel the insane love Cory and I had for each other morphed into this strength that I have right now.
"There's just something about knowing he's watching everything I'm doing and feeling like I have to do everything now not just for me but for him. I also have a safety net below me — if I fall or if it's too much, my friends and family will be there to catch me."
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