Bring 1 quart water to a boil in a large nonstick skillet. Add Brussels sprouts, and boil 5 minutes or just until tender; drain well. Keep Brussels sprouts warm. Wipe pan dry with paper towels.
Cook pancetta in pan over medium-high heat until crisp. Remove pancetta from pan; drain on paper towels.
Melt butter in pan over medium heat; cook 2 minutes or until lightly browned, stirring occasionally. Add Brussels sprouts; stir in balsamic vinegar, salt, and pepper. Cook 3 minutes or until balsamic vinegar is slightly thick and mixture is thoroughly heated. Stir in pancetta. Remove from heat; sprinkle with toasted walnuts.