Place potatoes and garlic in a Dutch oven and cover with cold salted water by 1 inch. Bring to a boil; reduce heat and simmer, covered, until just tender, about 15 minutes; drain well and return to the Dutch oven over low heat.
Stir with a wooden spoon until potatoes are dried out, about 1 minute.
Transfer potatoes to a large bowl. Using a potato ricer, rice potatoes and garlic. Stir in butter, sour cream and half and half, being careful not to overstir; season with salt and pepper, to taste.
Serve immediately, garnished with chives, if desired, or keep warm in the Dutch oven, covered, for 30 minutes.
Notes
Choose the right potatoes. Russet potatoes are a classic choice for mashed potatoes, yielding ultra-fluffy potatoes. Yukon golds, on the other hand, are naturally rich and buttery, and a great alternative here. You can also do a 50/50 mix for a balance of fluffiness and richness.
Start with cold water. Boil the potatoes in cold water to start, ensuring that they cook evenly from the inside out.
Use room temperature dairy. Using cold dairy products (ex. butter, sour cream, and half and half) will cool down and seize the potatoes.
A potato ricer is key. Skip the blender and food processor! A potato ricer will create lump-free, ultra-smooth, fluffy potatoes without making them sticky or gluey.
Gently reheat. When reheating leftovers, reheat on the stovetop over low heat, mixing gently until smooth and hot with a splash of cream or milk and a pat of butter.