Harry, Queen Elizabeth's grandson, and US actress Markle will tie the knot on Saturday in Windsor Castle's 15th-century St George's Chapel.
Ptak said her design was more of an "installation" than a typical wedding cake, but offered few details.
The couple decided to buck the royal tradition for fruitcake and instead opted for a lemon and elderflower creation with buttercream icing, topped with fresh flowers.
Harry and Meghan chose lemon and elderflower because they wanted to enjoy flavours that are in season, Ptak said.
Ptak is originally from California and worked at Chez Panisse in Berkeley under chef Alice Waters before moving to London. She started her own business cooking at home and selling cakes on a stall in east London's Broadway Market.
"It is obviously such an honour to be asked to do this because for me I have been baking since I was a little kid and this is my dream," she said.
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