Cooking these potatoes in very salty water seasons them and makes them easy to smash.
Adjust oven rack to upper-middle position and heat oven to 450 degrees. Set cooling rack inside one rimmed baking sheet.
Add water to large pot and bring to boil over medium-high heat. Carefully add salt to water and stir with slotted spoon until dissolved.
Carefully add potatoes to pot (ask an adult for help). Cook potatoes until very tender, 20 to 30 minutes. To check if potatoes are tender, use slotted spoon to carefully transfer one potato to wire rack in rimmed baking sheet. Carefully insert paring knife into potato. If it slips in and out easily the potatoes are ready. Turn off heat.
Use slotted spoon to carefully transfer potatoes to cooling rack set in baking sheet. Let potatoes sit to dry out for 10 minutes.
Meanwhile, spray second rimmed baking sheet with vegetable oil spray. Use pastry brush to evenly coat baking sheet with olive oil.
Use clean pastry brush to paint potatoes with half of vinegar. Sprinkle potatoes with pepper.
Use oven mitts to remove baking sheet from oven and place on cooling rack (ask an adult for help).
In this recipe, salt does double duty: not only does it give the potatoes their salty taste, but it also gives them their creamy texture. Just like in the ‘Tis the Season experiment, there’s a lot more salt dissolved in the supersalty boiling water than there is in the raw potatoes. As the spuds cook, some of that dissolved salt moves from the water into the potatoes, seasoning them all the way through. (Thanks, diffusion!) The salt also breaks down pectin in the potatoes (pectin is like a glue that holds the potatoes’ cells together). As the pectin breaks down, the cells inside the potatoes start to separate. At the same time, lots of tiny starch granules in the potatoes’ cells absorb water and swell up (think: teeny sponges), giving the potato a creamy (and smashable!) texture.