Begin by filling a medium pot with 8 cups of tap temperature water and 1½ teaspoons of kosher salt. Next, peel, rinse, and cut the potatoes into roughly the same-sized large pieces. They shouldn't be smaller than 2-inch (5cm) cubes. (see FAQs about how potato piece sizes affect texture/flavor).
Add them to the pot and bring the pot to a very gentle boil on your stovetop. You should see foam build up on the top of the water, but the water's surface shouldn't look like it's doing more than shifting a little. It’s okay to allow some bubbles while boiling but keep them gentle. You don’t want the potatoes moving around too much in the water or they will rub against each other and break apart before the centers can cook all the way through. Boil for between 30-45 minutes, until a fork slides into the center of the largest piece(s) without resistance (See FAQ for a more thorough description of doneness)
Once ready, drain the pot of potatoes over a colander. Give the potatoes 1-2 minutes to let off some steam. Then, using tongs to transfer the hot pieces of potato, push everything through a potato ricer back into the empty pot. Return the pot to the stovetop and turn the burner on warm/low. Let the potatoes sit for another 3-5 minutes to allow them to steam and dry out even more. To help release moisture, use a fork to gently fluff the potatoes every minute or so.
At this point, add just the butter. (Why just butter? See the FAQs!) Use a spatula to gently fold the potatoes around until the butter is completely melted. Then, add the rest of the salt, the pepper, and the sour cream. Fold again, until the sour cream is mostly incorporated and then mix in the milk, to thin everything out. Taste your homestyle mashed potatoes and add any additional salt, if needed.