1. Heat oven to 375 degrees. Butter and flour one very large or two small baking sheets, or line with parchment paper.
2. Pour water into heavy saucepan. Cut butter into small pieces and add to water. Add salt. Place saucepan over medium-low heat so butter melts before water boils. Bring water just to boil.
3. Remove pan from heat and add flour all at once, stirring vigorously with a wooden spoon until dough forms into a ball and bottom of pan is filmed with flour. Let dough rest 5 minutes.
4. Add whole eggs, beating in one egg at a time. Dough should be stiff but smooth.
5. Immediately drop 1/4 cupfuls of dough 3 inches apart on baking sheet. (Or for a neater appearance, use pastry bag with 3/4-inch plain tip and pipe dough onto baking sheet.)
6. Combine egg yolk and milk in a small bowl. Brush each puff with glaze mixture, taking care not to let liquid drip onto pan.
7. Bake 35 minutes, until puffed, golden brown and firm.
8. Cool puffs on wire racks, pricking each with a cake tester or toothpick to allow steam to escape, or leave them in a turned-off oven with the door propped open for about an hour, until firm. (If baked pastry is filled before cool and firm, it will be soggy and may collapse.) Baked puffs should have hollow, moist interiors and crisp outer shells that are lightly browned.
9. Cut off tops, fill with whipped cream (use pastry bag with star tip or scoop the whipped cream with a large spoon). Replace tops of the puffs and sprinkle with powdered sugar. Enjoy!