Preheat the oven to 400 degrees F.
Heat water and margarine or butter to boiling point. Add flour and stir constantly until mixture is smooth and forms a ball when tested in cold water. Remove from heat and let cool. Beat in 4 eggs, one at a time. Drop dough from teaspoon to form small eclairs onto greased cookie sheet. Bake for approximately 30 minutes or until light brown. Set aside to cool. Prepare filling by combining milk, sugar, salt and flour; cook slowly until thickens. Add 3 beaten eggs and continue to cook until mixture is even thicker. Remove from heat, cool and add vanilla.
With a serrated knife, slice pastry puffs lengthwise, but not all the way through. Pipe custard mixture into the center. Melt chocolate for icing, add sugar and cream. Cook over medium heat until soft ball stage. Let cool and beat until smooth. Ice tops of the eclairs.