Poke a few holes in the bag of Brussels sprouts and steam in microwave for about 2-2½ minutes, until soft. To cook on the cooktop: Add Brussels sprouts to a pot and add a little water. Cover pot and steam until soft, about 5 minutes. Drain and cut each in half.
Coat a large nonstick pan with olive oil cooking spray. Heat olive oil in pan. Add Brussels sprouts and garlic. Season with a little salt and pepper and a dash of cayenne, if desired. Sauté over medium-high heat. Continue to stir often. Use a spatula to turn over several times. Add balsamic vinegar and continue to cook until blackened.