Place the Brussels Sprouts on a baking sheet, including any of the loose leaves. Cut the Pancetta into 1/2-inch dices and add to the pan. Add the Olive Oil, the Kosher Salt and Freshly Ground Black 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 at the 20-minute mark.
Meanwhile, place Balsamic Vinegar in a small saucepan or frying pan and simmer gently until it is reduced by half and looking syrupy — be careful here! Err on the side of under reducing — once the balsamic starts thickening, it can burn quickly. At the first sign of the balsamic getting too thick or dark, transfer it to a small bowl.
Remove pan from the oven, drizzle immediately with the balsamic vinegar and toss again. Taste and adjust the seasoning with salt and pepper. Serve hot.