Bring a pot of water to a boil and cook the peeled and cubed russet potatoes until very tender, 15 to 20 minutes; the cubes should yield easily when pressed. Drain well in a colander and let steam off briefly so the potatoes aren’t watery.

Blanch the diced small carrots in boiling water for 2 minutes, until bright and just tender. Drain thoroughly.

Salt the thin cucumber slices and let them sit for 10 minutes; you should see moisture beads on the surface. Squeeze out the excess liquid so the salad stays creamy instead of watery.

Mash the warm boiled russet potatoes and leave some chunks for a slightly mashed texture, not fully smooth. Transfer to a large mixing area and let cool slightly so it doesn’t melt the mayonnaise.

Combine the mashed potatoes with the blanched carrots, squeezed cucumber, corn kernels, and chopped hard-boiled eggs. Stir just until evenly distributed; the vegetables should be visible throughout.

In a separate bowl, mix Japanese mayonnaise, rice vinegar, sugar, and salt and pepper until smooth and evenly blended. Taste and adjust salt and pepper so the dressing is balanced.

Fold the dressing into the potato mixture until creamy and glossy, with the mashed chunks still intact. Scrape the sides to fully coat the potatoes.

Refrigerate the salad for at least 2 hours (up to overnight); the texture should thicken and the flavors should meld. Cover well to prevent drying.