Place the potatoes in a large stew pot and add water until the potatoes are just covered. Turn the heat to medium high and allow the potatoes to boil for 15-20 minutes, or until they can be pierced easily with a fork. Strain the potatoes and then mash them with a potato masher until all of the potatoes are crumbled. Then add the butter, milk, sour cream, garlic powder and salt and continue to mash the mixture until all of the ingredients are thoroughly combined and the potatoes are light and fluffy. Serve immediately.