1 pound russet potatoes, peeled and quartered
2 packages (1/4 ounce each) active dry yeast
1-1/2 cups warm milk (110° to 115°)
1/2 cup canola oil
1/2 cup sugar
1 teaspoon salt
7-1/2 cups all-purpose flour
Additional oil for deep-fat frying
4 cups confectioners' sugar
1/3 cup water
1 teaspoon vanilla extract
Place potatoes in a large saucepan and cover with water. Bring to a boil. Reduce heat; cover and cook for 15-20 minutes or until tender. Drain, reserving 1/2 cup cooking liquid; cool to 110°-115°. Discard remaining cooking liquid. Mash potatoes without milk or butter.
In a large bowl, dissolve yeast in reserved cooking liquid. Add the mashed potatoes, milk, oil, sugar, eggs and salt. Add enough flour to form a soft dough. Place in a greased bowl, turning once to grease top.
Cover and let rise in a warm place until doubled, about 1 hour. Punch dough down; let rise again until doubled, about 20 minutes. Roll out on a floured surface to 1/2-in. thickness. Cut with a floured 3-in. doughnut cutter.
In an electric skillet or deep-fat fryer, heat oil to 375°. Fry doughnuts, a few at a time, until golden brown.
For glaze, in a large bowl, combine the confectioners' sugar, water and vanilla until smooth. Dip warm doughnuts in glaze. Cool on wire racks. Makes 4 dozen.