Place the broccoli in a large pot of boiling water, and blanch (cook) for 3 minutes or until the broccoli is tender but still crisp. Drain well, and return to the pot or a large bowl.
While the broccoli is cooking, melt the butter in a small skillet over medium heat. Mix in the brown sugar, soy sauce, vinegar and garlic. Bring to a boil, then remove from the heat. (For a quick option, you can heat the mixture in the microwave until the brown sugar has dissolved.)
Drizzle the sauce over the cooked and drained broccoli, and toss to evenly coat. Sprinkle with the cashews, and serve immediately.