Heat balsamic vinegar and sugar in a small pot or saucepan over medium heat.
Bring to a gentle boil, then reduce heat to medium-low heat and let simmer, stirring occasionally until the vinegar thickens and is reduced to about 1/2 cup (about 20 minutes). It should be thick enough to coat the back of a spoon.
Remove from heat and allow to cool completely before serving. If too thick reheat gently for a few seconds over the stove.
Preheat the oven to 400°F.
Trim the ends of sprouts and cut them in half lengthwise.
Roast the Brussels sprouts, turning once through cooking, until tender with charred edges around 20 to 25 minutes depending on your oven, be careful not to overcook them or burning the leaves.
In a bowl toss the sprouts with the balsamic vinegar. Adjust taste with extra salt and pepper, if needed.