Place peeled, cubed Yukon Gold potatoes and thyme sprigs into a large cold pot. Cover with cold water by 1 inch. Stir in 1 tablespoon of kosher salt. Bring to a gentle rolling boil over high heat, then reduce to medium and simmer for 15 to 18 minutes, until fork-tender and easily pierced without resistance.

Drain potatoes thoroughly in a colander, discarding the thyme stems. Return the drained hot potatoes back into the dry warm pot over low heat for 1 to 2 minutes, gently shaking the pot. This evaporates residual surface moisture so the potatoes absorb the butter and cream like a sponge.

While potatoes simmer, melt unsalted butter in a light-colored saucepan or stainless skillet over medium heat. Swirl constantly as the butter melts, foams, and crackles. After 3 to 4 minutes, the milk solids will turn a fragrant golden-hazelnut color and smell intensely nutty. Remove from heat instantly and stir in minced garlic—the residual pan heat will gently roast the garlic without burning.

Pass the hot steam-dried potatoes through a potato ricer or food mill into a large bowl (or mash thoroughly with a handheld masher until smooth). Pour in three-quarters of the warm nutty brown butter with garlic, followed by the warm heavy cream and milk. Fold gently with a silicone spatula until silken and luscious. Season with kosher salt and black pepper.

Transfer the mashed potatoes to a warm shallow serving bowl, creating billowy swoops with the back of a spoon. Drizzle the remaining reserved brown butter into the crevices, sprinkle generously with minced fresh chives, and finish with a pinch of crunchy flaky Maldon sea salt. Serve hot!