Add butter to boiling water. Heat until butter melts. Add flour and salt all at once, stirring vigorously. Cook, stirring mixture until it leaves the side of the pan. Remove from heat, cool 1 minute. Add eggs one at a time, beating with spoon after each egg until smooth. Drop by heaping tsp, 2” apart on greased cookie sheet, shaping into rounds. Bake in hot oven at 450° for 10 minutes then at 400° for 20 minutes. Turn off oven and make a slit in each one to let out the steam (so they don’t get soggy in the middle). Let stand in oven 5 minutes more. Allow to cool and cut each in half, frosting the top half with chocolate frosting. Whip whipping cream with sugar and fill each puff.