Peel and dice the russet potatoes into 1-inch chunks. Place them in a large pot filled with salted water and bring to a boil over medium-high heat. Cook until fork-tender, about 15 minutes. Drain well.

Return drained potatoes to the warm pot and let them steam dry for 1–2 minutes to eliminate excess moisture.

Mash the potatoes until smooth, then mix in butter, warm milk, salt, and black pepper until creamy.

Preheat oven to 375°F (190°C) and brush muffin tin wells with olive oil.

Spoon mashed potatoes into each muffin well, smoothing tops and creating a well in the center.

Sprinkle shredded cheddar cheese on top of each cup, filling the well. Brush edges lightly with olive oil.

Bake for 12–15 minutes until cheese melts and edges turn golden brown.

Let cool for about 2 minutes, then transfer to a serving platter and garnish with chives.