Step 1

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.

Step 2

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.

Step 3

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.