3 medium Yukon Gold potatoes, peeled and cut into 3/4-inch cubes
2 medium sweet potatoes, peeled and cut into 3/4-inch cubes
5 garlic cloves, peeled
1/2 cup low-fat buttermilk
1 tablespoon butter, cut into 4 pieces
Fill a large stockpot with 3 inches of water, place a steamer basket in pot, and bring water to a boil. Reduce heat to low; place all the potatoes and garlic cloves in steamer basket and cover. Cook until tender, about 15 minutes. Run potatoes and garlic cloves through a food mill (or mash by hand) into a medium saucepan set over low heat. Add 1/4 cup buttermilk and stir until incorporated. Add more buttermilk until desired consistency is reached. Toss in butter pieces, stir, and serve immediately.