Melt butter in a medium saucepan. Add water and bring to a boil. Add flour and salt all at once, stirring vigorously. Cook and stir until mixture forms a ball that doesn't separate. Remove from heat. Cool slightly, about 5 minutes. Add eggs, one at a time, beating with a wooden spoon after each, for 1 to 2 minutes. Drop batter by heaping tablespoon 3 inches apart onto greased baking sheet. Bake at 400 for about 30 minutes or until golden brown and puffy. Remove from oven, split, and remove any soft dough inside. Cool on wire rack. Fill with whipped cream, pudding, ice cream, or other fillings.