Peel and chop potatoes, then boil in salted water until tender about 12-15 minutes
Drain potatoes, mash roughly with a fork, and season with vinegar and pepper while still warm
Set aside to cool
Salt the cucumber and leave for 5-10 mins to draw out excess water, then squeeze dry
Boil the carrot and corn until just cooked
Mix together mashed potato, cucumber, carrot, corn, egg, spring onion, ham, mustard, and Kewpie until creamy and well combined
Chill for an hour for the best flavour
Top with extra spring onion and a crumble of boiled egg yolk to serve
