Peel the potatoes and then slice them lengthwise, then widthwise, so you have four large quarters.
Place the potato quarters into a pot, then cover them with cold water. (Just enough cold water to fully cover the potatoes.)
Bring the water to a boil and season it with some salt. Let the potatoes boil for about 15 minutes, or until they are fork tender. You can test them with a fork or knife. If the utensil goes in without any resistance, they're ready!
Drain the potatoes well, then roughly mash them up immediately. Add the butter, then mix the potatoes with a hand mixer or hand masher, mixing in the butter.
Add half of the milk, half of the Greek yogurt, and a generous pinch of salt. Continue to mix or mash and give it a taste. You'll likely need a bit more creaminess and salt. Add the rest of the milk and yogurt, and another generous pinch of salt.
Mix or mash until the potatoes are perfectly smooth; do another taste to ensure you've got enough salt.
Transfer the mashed potatoes to a serving bowl, top them with a pat or two of butter, and then garnish with cracked pepper and fresh chives.
These mashed potatoes will keep well in the fridge in an airtight container for about 3-4 days. You can also freeze them in an airtight freezer friendly container or bag for up to a year!