Mix the yeast and 1 teaspoon of sugar into the warm water. Allow the yeast 10 minutes at room temperature to develop and turn a little bubbly. Boil the potato in unsalted water until tender; drain off all the water. Mash the potato and measure 1 cup. Mix the 1 cup of mashed potato with the hot milk and mash it very smooth. Stir in the butter, ½ cup sugar and salt to the potato mixture. Once it has cooled to lukewarm, stir in the egg, then the yeast mixture, and then the flour. Knead on a floured board about 5 minutes, adding flour as needed. The dough will be smooth but still somewhat sticky. Place the dough in a buttered bowl, butter the top and loosely cover it with a piece of buttered plastic wrap. Leave it in a warm place to rise about 1½ hours.