The innuendo-filled gag came on the BBC special which saw the former Bake Off judge, 84, help the Duke and Duchess of Cambridge, both 37, prepare a menu for a charity event.
"Did Mary just ask Catherine if she would like to squirt on a cake?" one tweeted.
"Mary Berry asking Kate Middleton if she'd like to squirt on a cake is ICONIC," another wrote.
"Squirt on the cake, she's going to be queen!" one wrote.
Viewers loved that Kate was "fangirling" over Mary and they want the duo to have their own show.
"We grow out own vegetables," Kate told the famous British cook.
"We’ve got carrots, beans, beetroot - a massive favourite - Louis absolutely loves beetroot," Kate said of the 19-month-old.
As for Charlotte, four, she's all about a special variety of a humble staple. "Charlotte - potatoes ... obviously," Kate remarked.
As for the royal rascal, Prince George, six, the jury is still out on what vegetables he eats.