"I have the best time being your mum. I love our nightly evening chats when I really get to hear what's on your mind.
"You work hard to get whatever it is you want to achieve, and you have grit and responsibility in spades. I am so damn lucky to be your mother, you beautiful, kind young woman. Thank you for choosing me. I adore you to the moon and back a zillion times.
"I'm sorry you are having this particular birthday during these circumstances, but as always with you, you find the best in everything."
Paltrow also shares son Moses, 14, with Coldplay's Chris Martin.
The 16-year-old recently trolled her mum over her GOOP store's more unique items, writing up her own version of her Paltrow's quarantine to-do list.
"Make more vagina eggs & candles," she wrote.
Paltrow's wellness company Goop was notoriously endorsed egg-shaped jade and rose quartz stones to insert into the vagina for hormone balance and other health benefits.
Another of the store's products, a $75 candle titled "This Smells Like My Vagina" also went viral.