Dissolve yeast in warm water until foamy. In a large bowl, combine 1 cup buttermilk, 1/2 cup butter, egg, sugar, salt and yeast mixture, then add 3 cups flour; beat on medium speed until smooth, 1 minute. Add remaining flour, 1/4 cup at a time, to form a soft dough., Turn dough onto a lightly floured surface; knead until smooth and elastic, 6-8 minutes. Place in a greased bowl, turning once to grease the top. Cover and let rise in a warm place until doubled, about 1 hour., Punch down dough. Turn onto a lightly floured surface; divide and shape into 20 balls. Place in a greased 13x9-in. pan. Cover with a kitchen towel; let rise in a warm place until almost doubled, about 45 minutes., Preheat oven to 350°. Brush rolls lightly with remaining buttermilk and butter. Bake until golden brown, 20-25 minutes. Cool in pan 20 minutes. Remove to a wire rack; serve warm.