Bring a large pot of water to a boil and add kosher salt.
Add sweet potatoes to the boiling water, reduce heat to medium, and simmer until fork-tender, about 25 to 30 minutes.
Drain the sweet potatoes in a colander and return them to the pot or transfer to a large mixing bowl.
Use a potato masher to mash the sweet potatoes until mostly smooth. (This helps avoid a gummy texture.)
Add half of the butter, the whole milk, and the maple syrup to the mashed sweet potatoes. Mix using a hand mixer on low speed or stir well with a spatula until the mixture is smooth and creamy.
Taste and adjust seasoning with more kosher salt if needed.
Transfer the mashed sweet potatoes to a serving bowl. Garnish with pats of the remaining butter and a few sprigs of fresh thyme.
Serve warm.