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