Preheat the oven to 400 degrees F. Set out a sheet pan.
Prep The Potatoes. Wash and dry the potatoes. Prick them several times with a fork (to allow steam to escape as they bake). Rub the potatoes lightly with oil and sprinkle with kosher salt.
Bake The Potatoes. Place the potatoes on the sheet pan and bake for 60-70 minutes, or until the potatoes are fork tender. (A fork or sharp knife should easily pierce through the potatoes).
Slice & Scoop. Let the potatoes cool just 5-10 minutes, then carefully slice them in half lengthwise. Use a spoon to scoop most of the flesh out of the potatoes. Leave a small border (about 1/4-inch) around the inside to help the potato skins hold their shape.
Make The Mash. Working quickly, add the flesh to a large mixing bowl along with 2 Tablespoons butter, 2 Tablespoons sour cream, and 4 Tablespoons of milk, and a generous pinch of salt and pepper. Mix with a hand mixer until well blended and creamy, adding more butter, sour cream, or milk as needed. (You don't want the mash too runny)
Fill Potato Skins. Spoon the mashed potato mixture back into the potato skins and place on a baking sheet. Top the potatoes with cheese (and bacon, if using). (Note: If making these ahead of time, refrigerate at this point).
Bake Again! Bake the potatoes for another 10-15 minutes. (Add about 5-10 minutes more if you refrigerated the potatoes ahead of time.)
Garnish. Sprinkle with green onions or any desired garnishes and serve. Store leftover potatoes in an airtight container in the fridge for 2-3 days.