Preheat your oven to 400°F (200°C). Thoroughly scrub the six large Russet potatoes under cool running water, removing any dirt or blemishes. Dry the potatoes completely.

Rub the outside of each potato with the 2 tablespoons of olive oil or vegetable shortening, then sprinkle them evenly with the 1 teaspoon of Kosher salt (for coating).

Pierce each potato deeply four to five times using a sharp fork; this allows steam to escape. Place the prepared potatoes directly on the oven rack, or on a foil-lined baking sheet.

Bake for 50 to 75 minutes, or until the potatoes are tender when squeezed or easily pierced with a fork. Carefully remove the potatoes from the oven and let them cool on a wire rack for about 10 to 15 minutes, until they are cool enough to handle comfortably.

Reduce the oven temperature to 350°F (175°C). Holding one cooled potato firmly, use a sharp knife to slice off the top lengthwise, cutting about 1/4 inch from the top edge.

Carefully scoop out the fluffy potato flesh from the center of the shell, ensuring that you leave a sturdy, intact border of about 1/4 inch of potato attached to the skin all around the sides and bottom. Place the scooped potato flesh into a large mixing bowl and set the empty shells aside on a baking sheet. Repeat this process with all six potatoes.

Add the 4 tablespoons of softened butter, the 1/2 cup of whole milk or heavy cream, and the 1/2 cup of sour cream to the bowl with the mashed potato flesh. Using a hand mixer or masher, beat the contents until the mixture is light, fluffy, and smooth. Do not overmix.

Fold in 1 cup of the shredded sharp cheddar cheese, the 1 teaspoon of Kosher salt (for filling), the 1/2 teaspoon of black pepper, and half of the chopped fresh chives or green onions. Taste the filling and adjust the seasoning as needed.

Spoon the potato mixture generously back into the six empty potato shells, mounding the filling high above the shell rim.

Evenly sprinkle the remaining 1/2 cup of shredded sharp cheddar cheese over the top of the filled potatoes. If using, sprinkle a small pinch of paprika over the cheese for added color.

Place the filled potatoes back into the preheated 350°F (175°C) oven. Bake for 15 to 25 minutes, or until the filling is heated through completely and the cheese on top is melted, bubbly, and lightly golden brown.

Remove the twice baked potatoes from the oven and allow them to rest for 5 minutes before serving. Garnish the top of each potato with the remaining fresh chives or green onions immediately before serving hot.