Scrub each potato under running water, pat dry, and prick the skin 6 to 8 times with a fork.

Preheat the oven to 400 F (200 C). Wrap each potato first in a piece of parchment paper, then in aluminum foil. The parchment stops the skin from tasting metallic; the foil traps steam.

Place the wrapped potatoes directly on the middle oven rack and bake for 90 minutes. To mimic the light pressed shape of the street-cart version, place an inverted baking sheet on top of the potatoes during baking so they flatten slightly and split more easily later.

While potatoes bake, prepare all toppings and place each in a separate small bowl. Cold toppings should be ready before the potatoes come out.

Remove the potatoes, unwrap carefully (steam will escape), and transfer to serving plates. Slice each lengthwise about three-quarters of the way through, keeping the potato joined at the base. Push the ends gently toward each other to open a wide pocket.

Immediately add 1 tablespoon of the butter to each hot potato. Sprinkle half the shredded cheese into each and, using a fork, mash the butter and cheese into the fluffy interior along with the salt and pepper. Work while the potato is very hot so the cheese melts into strings.

Once the base is creamy, spoon the cold toppings across the top: corn, olives, pickles, cabbage, kidney beans, and Russian salad in colorful stripes.

Finish with a light drizzle of ketchup and mayo, if using. Serve immediately while the interior is hot.