Pierce potatoes with a fork; arrange on paper towels in microwave oven. Microwave potatoes at HIGH for 10 minutes, rearranging the potatoes after 5 minutes. Wrap potatoes in a towel; let stand for 5 minutes. Scoop out pulp; discard skins. Combine pulp, milk, 1 tablespoon brown sugar, and salt in a medium bowl; mash.
Combine 1 tablespoon brown sugar, softened butter, pecans, and cinnamon in a small bowl. Top each serving with pecan mixture.