Peel 520g of russet potatoes and cut into 1½in / 4cm pieces. Cover with cold water, add 2 teaspoons of kosher salt and bring to a boil. Gently boil for about 15 minutes, until a skewer slides through.
Meanwhile, boil 60g of thinly sliced carrot and ¼ cup of frozen corn for 5 minutes. Drain.
Cover 1 large egg with cold water, bring to a boil and simmer for 11-12 minutes. Cool in cold water, then peel and chop.
Toss 1 thinly sliced Persian cucumber with ½ teaspoon of kosher salt. Leave for 5 minutes, then squeeze out the liquid.
Drain the potatoes and mash lightly, leaving some chunks. While hot, mix in 1 tablespoon of unseasoned rice vinegar and black pepper to taste. Let cool.
Add the carrot, corn, cucumber, egg, and 2 slices of Black Forest ham, cut into thin strips. Fold in 6 tablespoons of Japanese mayonnaise.
