Peel and dice russet potatoes into 1-inch chunks, place in a large pot with salted water. Bring to a boil and cook until fork-tender, about 15 minutes.

Drain potatoes in a colander, then return them to the warm pot off the heat. Let them steam dry for 1–2 minutes.

Add softened butter, warm milk, and seasonings. Mash until smooth and creamy, adjusting salt and pepper as needed.

Preheat the oven to 375°F (190°C). Generously brush a muffin tin with olive oil.

Spoon or pipe mashed potatoes into each muffin well, packing gently. Create a small well in the center.

Top each cup with shredded cheddar, brushing the edges with olive oil for extra color.

Bake for 12–15 minutes, or until the cheese is melted and the edges turn golden brown.

Allow cups to cool in the pan for 2 minutes before transferring to a platter. Garnish with chopped chives.