Cover the cubed potatoes with cold water in a large pot and add a generous pinch of salt. Bring to a boil and cook until the potatoes are fork tender, about 20 minutes.
Drain and return the potatoes to the pot on the warm burner, so the residual heat cooks off excess moisture. Let stand for 5–10 minutes.
Mash the potatoes:
Transfer the potatoes to a large bowl. Add the butter and sour cream. Using a potato masher, mash the potatoes until smooth.
Add the garlic powder, all but a pinch of minced chives, and a pinch of white pepper and use a spoon to stir it into the potatoes. Taste and add more salt, if needed.
Add a few splashes of milk–mashing between additions–to help the potatoes reach your desired consistency. You may need just a tablespoon or a little more. It just depends on your preference.
Taste and season once more.
To serve:
Spoon the potatoes onto a serving platter and use the back of your spoon to smooth it out. Add the remaining pinch of minced chives on top. Enjoy!
Notes
If you only have cold butter, microwave it in 10 second increments until it's softened. It doesn't need to be completely melted.