Preheat the oven to 400°F. Scrub the potatoes well under cold water, pat them dry with a paper towel as best you can.
Place the potatoes on a baking sheet, pierce the potatoes all over with a fork, just to pierce the skin.
Brush with olive oil.
Bake for 1 hour to 1 hour and 15 minutes. Potatoes are done when a knife or fork will slide all the way through with no resistance. The potatoes will darken in color and some of the sugars may run out, this is normal. Let the potatoes cool for 10 minutes.
While the potatoes cool, make the butter. In a medium-sized bowl whip the butter with the brown sugar, cinnamon, vanilla, and salt until smooth, light, and fluffy.
Slice the potatoes, fluff with a fork, and add the whipped butter, serve and enjoy!
If you want to make more than 4 potatoes at a time it may take a little longer to cook.