Diaz, 48, appeared on a special episode of Barrymore's talk show, The Drew Barrymore Show, which aired on the actress' 46th birthday Monday.
Diaz called into the show via Zoom. The actress said she and Barrymore have spent "so many birthdays together" but were unable to celebrate in person this year due to the COVID-19 pandemic.
"Usually we get to actually be together," Diaz said. "We're not big gift givers, we're more about experience, so we always try to go someplace and do something with the girls and be together. Cook, and eat, and hang, and stay up all night and just have the best time."
Barrymore said Diaz is the only friend she'll let spoon her in bed. She also explained her "Poo Poo" nickname for Diaz, saying it started as "classic boy humor" and evolved into "a true term of endearment."
"When we're at a restaurant she'll say to the waiter, 'Ask Poo Poo what she wants,'" Diaz added. "And people will be like, 'What?' We call each other Poo Poo. I call her Poo Poo. She's in my phone as Poo Poo."
Diaz and Barrymore got emotional as they spoke about their friendship and close bond.
"It's truly one of the great joys of my life, is our friendship," Diaz said. "When I think about you, literally, I will cry, because I just think you're so special and to celebrate you is the absolute -- every year, it's one of my favorite times of the year is to get to celebrate you."
"You've made me a better person my whole life," Barrymore added. "And you've seen me through everything. Ugly, beautiful, loss, life, birth."
Barrymore and Diaz played Dylan Sanders and Natalie Cook in the 2011 film Charlie's Angels and the 2003 sequel Charlie's Angels: Full Throttle. Diaz and fellow Charlie's Angels star Lucy Liu were the first guests on Barrymore's talk show when it premiered in September.
"[We're a] tripod and the three little bears that met and became magnets and we're still here together," Barrymore told Entertainment Tonight.
"We have experienced life, love, death, birth, loss, gain, we have lived all of the different themes that life has in it, and so we want to explore that on this show," she added. "I thought, 'Who else better to do that with than two women I have been doing that with for 20 years?'"