Arrange a rack in the middle of the oven and heat the oven to 400°F.
Prepare 4 medium russet potatoes: Cut a cross shape about ¼-inch deep into each potato. Place the potatoes cut-side-up directly onto the oven rack. Bake until very crisp and crackly on the outside, about 2 hours.
Carefully remove the potatoes from the oven with tongs and place on a large plate, baking sheet, or heat-proof cutting board. Carefully cut deeper into the cut you made initially. Gently push the two opposite ends of each potato toward each other to open them, release steam, and expose some of the flesh.
Return the potatoes to the oven rack and bake for 10 minutes more. Push the two opposite ends of each potato toward each other again to open them fully. Top the flesh with a generous dollop of butter and season with flaky salt and freshly ground black pepper.