Peel and rinse the sweet potatoes. Cut into 2-inch pieces. Put them in a medium pot with cold water, and make sure the water covers the sweet potatoes completely. Season with a pinch of salt. Bring to a boil over high heat. Once the water starts to boil, simmer and cook until the sweet potatoes are fork tender, about 15 – 20 minutes.
Drain the sweet potatoes, and return them to the pot (no heat). Mashed them with a masher or large fork until there is no lump.
Meanwhile, heat the milk in the microwave for 45 seconds or until warm.
Add the butter and 1/3 cup warm milk into the mashed sweet potatoes, stir together. If needed, add more warm milk to reach a consistency that you prefer. Stir in brown sugar. Taste and adjust with more sugar if needed. Serve immediately.