Fill a large pot with cold water about halfway full. Add 2 tablespoons kosher salt to the water. Stir until dissolved.
Wash the potatoes well with a vegetable brush(do not peel). Cut into roughly 2-inch pieces, placing the pieces into the pot of cold water as you go.
Bring the pot to a boil over high heat. Then, turn the heat to low and simmer for about 30 minutes, or until potatoes are easily pierced with a fork.
Drain the potatoes in a colander, or by holding the lid carefully over the pot at an angle to pour the water out while keeping the potatoes in the pot.
Add the salted butter (6 tablespoons) to the potatoes. Place a few of the potatoes over the butter. Cover the pot, so the heat stays in and melts the butter. Allow to sit until the butter is very soft or all the way melted (about 3 minutes).
Use a potato masher to break up the potatoes a bit and make sure the butter is evenly dispersed. Add the milk and continue to mash until desired consistency is reached and everything is mixed thoroughly. Season to taste with salt (and pepper, if you want) and stir/mash to distribute the seasonings.
Serve immediately for best results, with more butter if desired.
Notes
Yukon gold potatoes yield the most buttery, creamy consistency. That said, I've used red and white potatoes with great results too. You can also use russet potatoes, but I recommend peeling them because their skins are a bit thicker and more bitter.
To prep ahead of time, cut the potatoes and place them in the pot of cold water. Then, store the pot in your fridge for up to 2 days. When you're ready to cook, just put the pot directly on the stove and go from there!
The provided nutrition information does not include any added sodium from seasoning to taste, any optional ingredients, and it does not take brands into account. Feel free to calculate it yourself using this calculator or by adding the recipe to Yummly.