Trim and cut the broccoli florets into bite-sized pieces. Place a steamer basket on the bottom of a large pot. Then add the broccoli florets and about an inch of water. Cover and bring to a boil. Cook for about 7 minutes, or until tender but still crisp.
While the broccoli is steaming, melt the butter in a small skillet or saucepan over medium heat. Stir in the brown sugar, soy sauce, vinegar, pepper, and garlic. Bring to a boil, then remove from heat. Mix in the cashews, if desired.
Pour the garlic butter sauce over the broccoli and serve immediately.