Peel and cut potatoes into a ½-inch dice. To prevent potatoes from wobbling as you dice, cut a flat base for the potato to rest on as you make your slices. (Note: Potatoes can be peeled and diced 1 day in advance. Store completely submerged in cold water in the refrigerator, changing the water every few hours to prevent browning.) Place potatoes in a pot filled with cold salted water (as "salty as the sea"). Bring to a boil over high heat, 6–8 minutes. Once boiling, reduce to a simmer and cook until fork-tender but not falling apart, 15–20 minutes.
Drain the potatoes in a colander, then add back to the pot. Turn heat to low and let the potatoes release additional moisture, 1–2 minutes. Turn off the heat.Transfer potatoes to a large bowl. Work all the potatoes through the ricer and back into the pot.
With the pot over low heat, gently fold in butter and sour cream, mixing just enough to melt and incorporate; overmixing will result in a gluey texture. Add the milk a splash at a time, gently mixing to incorporate; repeat until you reach your desired consistency. (You may have milk left over.) Season to taste with salt and pepper. Transfer to serving bowl; garnish with extra butter and/or freshly chopped chives, optional. Serve immediately.