Because a small amount of pancetta adds lots of flavor, you don't need to use much, which keeps down the fat and calories. Browning the butter gives the dish a nutty flavor; watch carefully, though, since butter can burn easily.
1. 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.
2. Cook pancetta in pan over medium-high heat until crisp. Remove pancetta from pan; drain on paper towels.
3. 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.