Preheat the oven to 400 F.
Add the brussels sprouts, oil, salt, garlic powder and black pepper to a bowl and mix well.
Place the brussels sprouts on a baking sheet, cut side down.
Roast for 30 minutes.
Meanwhile, add the balsamic vinegar, maple syrup and rosemary to a small saucepan and bring it to a boil. Reduce to a simmer and stir occasionally for 10-15 minutes or until the sauce is thick enough to coat the back of a spoon.
Drizzle the reduced balsamic over the brussels sprouts and serve hot.