In a large saucepan, bring water, butter, sugar and salt to a boil over medium heat. Add flour all at one; stir until a smooth ball forms. Remove from the heat; let stand for 5 minutes. Add eggs, one at a time, beating well after each addition. Beat until smooth and shiny.
Insert a large round tip into a pastry bag or heavy-duty resealable plastic bag; add batter. Pipe into 12 strips (about 3 in. long) 3 in. apart on a greased baking sheet.
Bake at 400° for 30-35 minutes or until golden brown. Remove to a wire rack. Immediately cut a slit in each for steam to escape; cool.
For filling, combine the sugar and cornstarch in a small saucepan; gradually add milk until smooth. Bring to a boil over medium heat until thickened and bubbly. Reduce heat; cook and stir 2 minutes longer. Remove from the heat.
Stir 1 cup hot filling into egg yolks; return all to pan. Bring to a gentle boil; cook for 2 minutes, stirring constantly. Remove from the heat; stir in butter and vanilla. Cool.
Split puffs; remove and discard soft dough from inside. Fill eclairs just before serving. Replace tops; dust with confectioners' sugar or spread with frosting. Refrigerate leftovers. Yield: 1 dozen.