In a large bowl, stir 2 cups of the flour, 1/3 cup sugar, the salt and yeast with wooden spoon until well mixed. In 1-quart saucepan, heat water and 1/3 cup butter over medium-low heat, stirring frequently, until very warm and an instant-read thermometer reads 120°F to 130°F. Add water mixture and egg to flour mixture. Beat with wire whisk or an electric mixer on low speed 1 minute, stopping frequently to scrape batter from side and bottom of bowl with rubber spatula, until smooth. Beat on medium speed 1 minute, stopping frequently to scrape bowl. With wooden spoon, stir in enough of the remaining flour, 1 cup at a time, until dough is soft, leaves side of bowl and is easy to handle (dough may be slightly sticky).