In a large saucepan, bring the water, butter and salt to a boil over medium heat. Add flour all at once; stir until a smooth ball forms. Remove from heat; let stand for 5 minutes. Add 1 egg at a time, beating well after each addition. Continue beating until mixture is smooth and shiny.
Drop by 1/4 cupfuls 3 in. apart onto greased baking sheets. Combine milk and egg yolk; brush over puffs. Bake at 400° until golden brown, 30-35 minutes. Remove to wire racks. Immediately cut a slit in each for steam to escape; cool.
In a large bowl, beat cream until it begins to thicken. Add sugar and vanilla; beat until almost stiff. Split cream puffs; discard soft dough from inside. Fill the cream puffs just before serving. Dust with confectioners' sugar. Refrigerate leftovers.
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