Preheat oven to 200 degrees C.
Arrange the asparagus on a baking sheet. Coat with cooking spray, and season with salt and pepper.
Bake asparagus 12 minutes in the preheated oven, or until tender.
Melt the butter in a saucepan over medium heat. Remove from heat, and stir in soy sauce and balsamic vinegar. Pour over the baked asparagus to serve.