Place the potatoes into a large pot and cover with cold water by about 1 inch. Add the kosher salt.
Bring to a boil over medium-high heat, then immediately reduce to a gentle simmer. Cook for 18–20 minutes until the potatoes are fully fork tender.
Drain thoroughly and return the potatoes to the hot empty pot over low heat for 30–60 seconds, gently shaking occasionally to remove excess surface moisture.
In a small saucepan combine the heavy cream, whole milk, and garlic clove. Warm gently over low heat until steaming lightly. Do not boil. Remove and discard the garlic clove.
Pass the hot potatoes through a potato ricer into a warm bowl or back into the pot.
Add the cold butter gradually, gently folding until fully melted and incorporated.
Slowly fold in the warm cream mixture until smooth, fluffy, and creamy.
Fold in the crème fraîche and black pepper.
Season with the sea salt to taste.
Transfer to a warm serving bowl and top with melted butter, chives, and flaky sea salt if desired.
Serve immediately while hot and silky.