Kewpie is a brand of Japanese mayo that is richer and more yellow than American mayo, thanks to the super-orange egg yolk magic famous in Japan. US mayo uses whole eggs to make mayo, Kewpie uses all egg yolks. It also brings a sweeter note due to using sweeter vinegars than regular mayo in the US, which mostly relies on white distilled vinegar. If you can't find it in any of your local grocery stores or your Asian supermarket, you can make a similar mayonnaise substitute by combining 1 cup of regular mayonnaise with two tablespoons of rice vinegar and one tablespoon of sugar. Mix well and use for this recipe.