In a saucepan, melt 1/2 cup butter or margarine in 1 cup boiling water. Add 1 cup all-purpose flour and 1/4 teaspoon salt all at once; stir vigorously. Cook and stir until mixture forms a ball that does not separate. Remove from heat; cool slightly. Add 4 eggs, one at a time, beating until smooth after each. Drop level tablespoons of dough 3 inches apart on greased baking sheet. Bake at 400ºF. for 25 minutes. Cool on rack. Cut off the top third of each puff. Remove any soft dough from inside. Yield: 35 to 40 puffs.