Mix the apples and brown sugar in a small bowl. Set aside. Pierce the squash several times to let steam escape. (If you don't, the squash will explode while cooking.) Microwave on high until tender, about 5 minutes. Turn after 3 minutes to ensure even cooking. (If you don't want to use the microwave, bake the squash in a 350 degree oven for 15-25 minutes, or until it can easily be pierced with a fork.) Cut in half (careful, it will be hot.) Scrape out seeds. Fill the hollow of each half with the apple and brown sugar mixture. Return the squash to the microwave and cook until the apples are softened, about 2 minutes. (Or return to the 350 degree oven for 5-10 minutes.) Serve each squash half topped with a teaspoon of butter. Serve immediately!