Preheat oven to 350°F/180°C, combi steam setting. If your oven has variable steam settings, use 60%. If not, don’t worry! Just set to combi steam at the correct temperature and the oven will work out the humidity for you.
Prick the potatoes all over with a fork and place on a baking tray. Cook until they’re tender all the way through (a small knife should have no resistance when inserted into the center), 45 minutes to an hour. Remove and allow to cool for about 15 minutes. Increase oven temperature to 430°F/220°C (same settings).
While the potatoes cool, put the bacon onto another tray and cook until it’s golden and sizzling, about 10 minutes. Remove and reduce oven heat back to 350°F/180°C.
Cut the potatoes in half across the widest part, so they sit fairly flat when opened out. Scoop the flesh from each half, leaving a 3/8” (1cm) border so they don’t collapse. If you break or make a hole in any of the skins, don’t panic as you’ll be able to squash it back into shape later. Put the skins back into the baking tray (you may need to use a second tray if the first one won’t fit all the cut halves).
Mash the scooped out potato flesh with the sour cream, mustard, horseradish (if using), and butter. Stir through 2/3 of the cooked bacon (reserving the rest to sprinkle over the top), the scallions and corn kernels, the season everything well with black pepper. I don’t add salt to this mixture as we find the bacon salty enough, but you do what you like.
Spoon the potato mixture back into the skins, distributing it evenly between all of them. Scatter with the cheese and bake until the potatoes are hot through and the cheese is bubbling and golden, about 15 minutes. Serve hot.
Leftovers will keep for 2-3 days in the fridge and they reheat brilliantly, but this dish doesn’t freeze well.