Preheat oven to 375°.
In a saucepan, bring water, butter, and salt to a rolling boil; remove from heat.
Whisk in flour and stir until dough comes together; add eggs one at a time, beating thoroughly after each addition until batter is smooth.
Drop batter by tablespoonfuls onto a greased or parchment-lined baking sheet; use wet fork tines to fluff up each mound of dough.
Bake at 375° for 10 minutes; reduce oven temperature to 350° and continue baking for another 15 minutes until pastry puffs are light golden-brown; turn off oven but do NOT open oven door; after several minutes remove puffs from oven and slice; return puffs to baking sheet then return sheet to oven; allow puffs to rest several minutes inside oven with oven door slightly ajar.
Stir together sugar, flour, cornstarch, and salt in a saucepan; add milk, vanilla, and egg yolks; cook over low-medium heat, stirring, until thick; cool in refrigerator.
Whip cream until stiff and fold it gently into cooled filling mixture; spoon or pipe filling into pastry puffs; sprinkle powdered sugar over top.
NOTE: The unfilled puffs freeze well, may later be filled with your favorite chicken or ham salads, and can be made small-sized for appetizers ~ I've often enjoyed them this way at wedding showers :) .