Serves 4

Adjust oven rack to center position. Place two large sheets of heavy duty aluminum foil on a work surface. Working one sheet at a time, place half of potatoes in center along with 3 sprigs of thyme. Fold up foil and crimp edges to seal tightly. Repeat with other half of potatoes. Transfer pouches to a rimmed baking sheet and place in the oven. Set oven to 300°F. Roast until a thin skewer inserted into the potatoes shows no resistance, about 2 hours. Remove from oven and set aside until cool enough to handle.

Meanwhile, melt butter in a medium saucepan over medium-low heat, swirling gently until particles are pale golden brown and smell nutty. Immediately transfer to a large bowl or the work bowl of a stand mixer fitted with a whisk attachment. Add maple syrup. Pick leaves off of remaining 2 thyme sprigs, roughly chop, and add to bowl.

Peel sweet potatoes and discard skins. Add flesh to bowl with butter and syrup. Beat with the whisk attachment or a handheld mixer until smooth and fluffy, about 2 minutes. Season to taste with salt. Serve. See note above for make-ahead suggestions.