Sarah Michelle Gellar has remembered her "little sister" Michelle Trachtenberg on what would've been her 40th birthday

The 48-year-old star - who first met Michelle on the set of soap opera All My Children in the early 1990s - captioned her post: "When you turned 16, it was hard not to still see you as the little girl I met on All My Children. When you turned 21, I had to teach myself to not see you as the little sister anymore."

Sarah added: "I can't imagine what 40 would have been like. This I know ... it would be filled with love. Happy birthday @michelletrachtenberg."

Michelle played the role of Dawn Summers - the younger sister of Sarah's title character - in Buffy the Vampire Slayer from 2000 to 2003 and was said to be in talks for the planned revival of the show.

A source told DailyMail.com: "Michelle Trachtenberg was in talks to return to Buffy the reboot and she was incredibly hopeful about it."

It was said that Sarah was keen to bring her back for the highly anticipated sequel series as "the reboot would need her as much as it needed Sarah".

The insider added: "It is still in early stages, but the reboot will feel her void.

"Fans were hopeful that she would be cast and Michelle expressed interest and excitement at the opportunity."

20th Television - which is the studio behind the upcoming Buffy reboot - had issued a statement paying tribute to Trachtenberg after her passing was announced.

The studio told People magazine: "Michelle touched a generation of television fans throughout her career, including in her unforgettable role as the independent and strong-willed Dawn Summers in Buffy.

"She brought depth, heart, and authenticity to every performance and will be remembered for her remarkable talent.

"Our sincere condolences go out to her family and friends - she will be deeply missed."

Sarah had paid tribute to Michelle following her death by posting a series of photos of the pair together on the set of the show.

Quoting the season five finale, she wrote in the caption: "Michelle, listen to me. Listen. I love you. I will always love you."

She added: "The hardest thing in this world, is to live in it. I will be brave. I will live... for you."