Preheat oven to 400°F. In small pot, reduce balsamic vinegar to half its volume. It should become thick and syrupy.
Place brussels sprouts on baking sheet, including some of the loose leaves, which get crispy when they're roasted. Cut pancetta into 1/2-inch dice and add to pan. Add olive oil, 1 1/2 tsp. salt and 1/2 tsp. pepper and toss with your hands. Spread out the mixture in a single layer.
Roast brussels sprouts until tender and nicely browned and pancetta is cooked, 20 to 30 minutes, tossing once during roasting. Remove from oven, drizzle immediately with balsamic vinegar syrup and toss again. Taste and adjust seasoning with salt and pepper. Serve hot. Serves 6.