- Preheat the oven to 200°C. Arrange the potatoes on a sheet pan and bake for 30 minutes, until a knife goes through the potatoes easily. When the potatoes have cooled, peel and set aside.
- Melt the butter in a saucepan over medium heat. Saute the onion for about 1 minute, until they turn translucent. Add the pork and cook, stirring constantly, for about 5 minutes, until the pork cooks through. Transfer to a large mixing bowl when cooked.
- Add the potatoes to the pork and mash together until just combined. Add the soy sauce, sugar pepper and mash together. Add the heavy cream and mash until all the ingredients are just combined.
- Transfer the potato mixture to a sheet pan and spread it out to make a flat disk about ½ inch thick. Place in the refrigerator uncovered to cool and firm up for about 1 hour.
- While the potato mixture is cooling, prepare 4 plates. Pour the flour onto the first plate, beaten eggs, panko into the subsequent plates separately. Pour the beaten egg onto the second plate and pour the panko onto the third plate. On the fourth plate, leave it empty as this plate will hold the breaded korokke.
- Place a cast-iron skillet on a burner. Fill the skillet with vegetable oil to a height of at least 1 inch. Attach a deep-fry (or “candy”) thermometer to the side of the skillet. On a work surface near the skillet, set up a tray lined with paper towels to drain the cooked korokke.
- When the potato mixture is ready, remove from the refrigerator and cut into 8 even pieces. Form 8 korokke patties, one at a time. Wet your hands, then place one of the pieces of the cooled potato mixture onto your palm. With both hands, gently form it into a flat, oval patty. Repeat for the remaining patties.
- Turn on the heat under the skillet to high. Heat the oil to 170°C.
- While the oil is heating, bread the korokke patties one at a time. First, dredge a patty in flour on both sides and shake off the excess flour. Second, dip the patty in the egg, coating both sides. Finally, lay the patty on the panko crumbs. Pile panko on top of the korokke with your fingers, then gently press the panko onto the patty so a generous layer of panko sticks on both sides. Be careful to handle the korokke very gently so it doesn’t break apart. Place the completed korokke on the empty plate. Repeat with the remaining patties.
- When the oil has reached 170°C, carefully slide the korokke into the skillet. Deep-fry to korokke in batches. Regulate the heat to maintain a constant 170°C oil temperature. If the oil is too hot, the korokke will burn; if too low, the korokke will come out soggy and greasy.
- Cook the korokke for about 3 minutes, until they heat through and turn golden brown (they’re already cooked inside). When they’re ready, transfer the korokke to the paper-lined plate to cool.
- Serve immediately, with tonkatsu sauce.