Cairns, who started her business with husband Kishore Patel 25 years ago, has been commissioned to create a traditional multi-layered fruit cake with a floral design for the royal wedding next month, along with a second one made from rich tea biscuits, reports the Telegraph.
Middleton has given detailed plans for the cake, which will be the centrepiece of the Buckingham Palace wedding reception.
The cake will be decorated with William and Middleton's new cipher - thought to feature the couple's entwined initials - which will be officially released on their wedding day.
Another theme will be the four flowers of the home nations - English rose, Scottish thistle, Welsh daffodil and Irish shamrock.
The Prince, however, has also requested his childhood favourite treat - a McVities chocolate biscuit cake.
Cairns, 56, was contacted by William's office in February and asked if she would make the couple's wedding cake.
"It's multi-tiered, doesn't have colour - it's cream and white (icing) - and it's a traditional cake but also quite delicate and modern, all the tiers will have a different theme," she said.
The commission has been a process of discovery for Cairns who has also been asked by Middleton to feature around 16 different blooms and foliage for their meaning - known as the 'language of flowers'.
She also wanted elements from the Joseph Lambeth technique of cake decoration, where intricate piping is used to make three-dimensional scrollwork, leaves, flowers and other adornments.
The cake designer said Middleton made her feel relaxed when they met around six weeks ago at Clarence House to discuss ideas.
"She has guided us right from the beginning and has quite strong ideas. That makes it much easier than a bride who has absolutely no idea whatsoever, which has happened in the past," said Cairns.
"But she knew very much what she wanted and she brought us mood boards and told us what influences she would like us to use on the cake," she added.
The official cake is believed to include a range of produce from dried fruits like raisins and sultanas to walnuts, cherries, grated oranges and lemon, French brandy and free range eggs and flour.