The Oscar-winning actress joined Camilla on a visit to the former home of Queen Victoria on the island, and the pair could not resist the sweet treats as they basked in temperatures of close to 30C (86F).
Both appeared to opt for a whipped vanilla cone, complete with chocolate flake, and the duchess was so keen to tuck in she managed to drop some of it down her chin.
Fortunately, Dame Judi – so used to having to clean up messes made by the various incarnations of James Bond – was on hand with a tissue to help her wipe it away.
Dame Judi, 83, was an ideal tour guide for the trip to Osborne House, as she is a patron for its charity and filmed there two years ago for Victoria And Abdul, in which she played the title role.
It was the second time she had played Queen Victoria on the big screen, having won a BAFTA for her performance as the monarch in the 1997 drama Mrs Brown.
She showed Camilla, 71, the restored Durbar Room at the famous property, which was purchased by Victoria and her husband Prince Albert in 1845 and went on to become one of her favourite family homes.
“It is impossible to imagine a prettier spot,” she once said of the house, which is next to a private beach she also owned.
Following her death in 1901, the house was given to the state and parts of the pavilion were opened to the public for the first time three years later.
It is now looked after by English Heritage and is open all year round.
Following the visit to Osborne House, Camilla visited Cowes town centre and took part in a naming ceremony for a new high-speed passenger ferry from Red Funnel.
She then attended a reception at the Royal Yacht Squadron in the town.