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. Remove from heat and let cool. Beat in 4 eggs, 1 at a time and once incorporated, place into a pastry bag. On a greased cookie sheet, pipe out dough in thick lines that form eclairs. Bake for approximately 30 minutes or until light brown. Set aside to cool.
In a medium saucepan, prepare filling by whisking together the flour, cocoa powder, sugar and salt. Whisk in about 1 cup milk and cook over low heat, stirring often, until thickened or the consistency of mayonnaise. Temper in 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.