Combine water, butter, and salt in top of a double boiler. Cook over boiling water until butter melts. Add flour, all at once, stirring vigorously over boiling water until mixture leaves sides of pan and forms a smooth ball. Remove from heat, and cool slightly.
Add eggs, one at a time, beating well after each addition; beat until batter is smooth. (Batter will be slightly thin.) Spoon batter into a pastry bag fitted with a 1/2-inch plain tip.
Pipe batter into 4- x 3/4-inch strips on greased baking sheets. Bake at 400° for 25 minutes. Remove from oven, and cool baked pastry completely on wire racks away from drafts.
Spoon Crème Pâtissière into pastry bag fitted with a 1/4-inch plain tip. Make a small hole in both ends of baked pastry. Pipe Crème Pâtissière into one end. Frost top of each éclair with Chocolate Butter Frosting. Chill.