To cook 5 pounds sweet potatoes (about 6 medium) in the microwave, pierce sweet potatoes several times with tines of a fork. Place in a large microwave-safe bowl; cover with damp paper towels. Microwave at HIGH 15 minutes or until tender, turning every 5 minutes. Let stand 15 minutes. Peel and mash sweet potatoes with a potato masher or a handheld mixer until smooth. Stir in 1/2 cup butter, cut up; 1/2 cup maple syrup; 1/2 cup firmly packed brown sugar; 1 teaspoon ground cinnamon; 3/4 teaspoon ground nutmeg; 3/4 teaspoon ground ginger; and 1/2 teaspoon salt. Stir until butter melts. Transfer to a 4-quart slow cooker. Cover and cook on LOW 2 hours or until thoroughly heated; serve immediately. Sweet potatoes may be kept warm on LOW up to 4 more hours, stirring every 30 minutes.