Scrub the potatoes with a brush, then pierce them all over with a fork so steam can escape during cooking. Bake on a sheet pan for 1 hour, until a butter knife can easily be inserted into the potato.
When the potatoes are cool enough to handle, cut each one in half. Scoop out the flesh with a spoon (make sure not to scoop out too much, or the skin won’t be able to retain its shape), and place into a bowl.
Put the potato skins back in the oven to keep them warm. Mash up the scooped potato with a big fork or a potato masher, and stir in the buttermilk, yogurt, and cheddar cheese. Taste, and add desired amount of salt and pepper. Scoop this mixture into the potato skins.
If desired, sprinkle a little bit more cheddar on top, and broil the tops for a minute or two. Garnish with scallions, and serve.