Dissolve yeast in 1/2 cup of water with the 1 teaspoon of sugar. Set aside to rise. Heat buttermilk, butter and 4 tablespoons sugar, putting them in a mixing bowl. Add mashed potatoes, salt, eggs and 4 cups of the flour. Add yeast mixture. Add enough additional flour to make soft dough. Let rise until doubled. Punch down and let rise again until doubled. Put on floured board, divide into 6 balls, let rise. Roll each one out into a circle and brush with melted butter. Cut into 8 wedges and roll each up into a horn. Put on greased pan to rise. Mix yolk of egg with milk and brush on the horns. Sprinkle top with poppy seeds if desired. Bake at 375 degrees for 20 to 25 minutes until golden brown.