Begin by peeling the potatoes. Once peeled, dice them into approximately 1-inch chunks.
Place the diced potatoes into a large pot and fill it with enough water to cover the potatoes. Place the pot on medium-high heat and bring the water to a boil. Once boiling, reduce the heat to a simmer. Cook the potatoes uncovered for about 10 minutes, or until they can be easily pierced with a fork.
Once the potatoes are cooked, drain the water using a colander.
Immediately transfer the drained potatoes to the bowl of a stand mixer (if you don’t have a stand mixer, you can keep them in the pot). Mash the potatoes using a masher or the paddle attachment of the stand mixer on low speed. It is crucial to mash the potatoes while they are still hot to prevent them from turning gummy.
Add butter, sour cream, half of the milk, salt, and ground black pepper to the mashed potatoes.
Switch the stand mixer to medium speed and whip the potatoes until they are just smooth. Gradually add the remaining milk as needed. Be cautious not to overwhip the potatoes, as this can make them gluey.
If not serving immediately, cover the bowl with a lid or plastic wrap to keep the mashed potatoes warm. When ready to serve, garnish with chopped chives.