Place the Brussels sprouts in a large bowl and pour the reserved bacon fat over them, tossing gently a few times to evenly coat them.
Pour the sprouts onto a lined baking sheet in a single layer and bake for 35-40 or until they are lightly browned, tossing them once or twice after about 20 minutes to evenly brown them.
While the Brussels sprouts are roasting, make the Balsamic Reduction: Place the balsamic vinegar in a small saucepan and bring to a boil. Reduce the heat to a lower simmer and continue simmering for 10-15 minutes or until it has thickened (see Note).
Transfer the roasted Brussels sprouts to a serving platter and sprinkle with the bacon and cranberries. Lightly drizzle the balsamic reduction over them (don't use too much, it's potent stuff!).
Be careful not to over-cook the balsamic reduction otherwise it will become very thick and cool to a rock hard mass. If this happens simply add a little more vinegar to the saucepan, reheat and re-thicken.