Place the sweet potatoes in a large pot and cover them with about 1 inch of cold water.
Bring the water to a boil over high heat.
Reduce the heat to medium and cook for 15 to 20 minutes, or until the sweet potatoes are very tender.
Drain the sweet potatoes well in a colander.
Add the butter to a small skillet over medium heat. Cook for 4 to 6 minutes, swirling the skillet occasionally, until the butter foams and the milk solids turn golden brown.
Remove the browned butter from the heat when it smells nutty and toasted.
Transfer the drained sweet potatoes to a large mixing bowl.
Mash the sweet potatoes until mostly smooth.
Add the browned butter, warm milk, maple syrup, salt, cinnamon, nutmeg, and black pepper. Stir until smooth and creamy.
Taste and adjust the salt if needed.
Notes
Tips and Tricks
Watch the butter closely - Brown butter can go from golden to burned quickly. Remove it from the heat as soon as the milk solids are browned.Drain thoroughly - Extra water left on the sweet potatoes can make the finished mash loose.Warm the milk - Cold milk can cool the potatoes and make it harder to serve them hot.Mash to your preference - Leave a few small pieces for a more rustic texture or mash longer for a smoother side.
Storing and Reheating
Refrigerator - Store leftovers in an airtight container in the refrigerator for up to 4 days.Microwave - Reheat individual portions in short intervals, stirring between each one until heated through.Stovetop - Reheat over low heat, stirring occasionally. Add a small splash of milk if the potatoes have thickened.Make ahead - Prepare the mashed sweet potatoes up to 2 days in advance. Refrigerate, then reheat before serving.