Speaking in an interview with the Sunday People's Take It Easy magazine, the baking queen described the meeting, which took place at the Buckinghamshire offices of Child Bereavement UK, as "surreal".
"Prince William said Kate used my recipes," the excited cook added. "They were big fans at a charity auction."
The queen of the Victoria Sponge, who tastes scores of baking delights on The Great British Bake Off, also revealed how she keeps so trim ahead of the fourth series of the show, which begins on Tuesday on BBC2.
'I do drink hot water,' she told the Sunday magazine. "But I also eat smaller portions. People offer you quiche that is low fat but it tastes disgusting.
"I would rather have the real thing or a beautiful cake made with all the right ingredients - chocolate, butter and sugar - but a smaller slice."
It was the first time that the Great British Bake Off judge had spoken about meeting the Duke and Duchess of Cambridge back when Kate was five months pregnant.
William joked at the time that his mum-to-be wife – who gave birth to their baby son George in July – could do with "a few lessons" in cooking from the celebrity food writer.
"My stomach is a big fan of yours," he told Mary, 78, asking her: "Do you do chocolate brownies? I love chocolate brownies."
The prince then admitted "actually it's me who really needs lessons, to be honest!"
It looks like new mum Kate may try her hand at a spot of baking, now that she is enjoying time at home with her little prince. She confessed to being a big fan of the doyenne of baking, telling her in March: "I love your cookbooks and I love your cakes, they're so delicious."
The Great British Bake Off returns to our screens on Tuesday at 8pm on BBC2.