Harry and Meghan asked Ms Ptak to create a tasty treat incorporating the bright flavours of spring.
Some 200 Amalfi lemons were used, as well as 10 bottles of Sandringham elderflower cordial, made with plants from the Queen’s estate in Norfolk.
Ms Ptak and her team of six bakers have been in the large kitchens of Buckingham Palace throughout the week.
Meghan is a long-time fan of the California-raised chef and interviewed her for her now-defunct lifestyle blog, The Tig.
The chef said: “I can’t tell you how delighted I am to be chosen to make Harry and Meghan’s wedding cake. Knowing that they really share the same values as I do about food provenance, sustainability, seasonality and most importantly flavour, makes this the most exciting event to be a part of.”
Ms Ptak, a pastry chef under the feted Alice Waters at Chez Panisse in Berkeley, California, eventually set up her own business with a stall at east London’s trendy Broadway Market.
She now owns the Violet bakery in Hackney.
Kensington Palace said Harry and Meghan were “very much looking forward to sharing this cake with their wedding guests”.