Preheat oven to 400 degrees F (205 degrees C) and place rack in center of oven. Line a baking sheet with parchment paper. 

In a bowl sift together the flour, sugar and salt. Set aside. 

Place the butter and water in a heavy saucepan over medium heat and bring to a boil. Remove from heat and, with a wooden spoon or spatula, quickly add the flour mixture. Return to heat and stir constantly until the dough comes away from the sides of the pan and forms a thick smooth ball (about a minute or two). Transfer the dough to your electric mixer, or with a hand mixer, and beat on low speed a minute or two to release the steam from the dough. Once the dough is lukewarm start adding the lightly beaten eggs and continue to mix until you have a smooth thick paste. Spoon or pipe 12 mounds of dough onto the baking sheet, spacing them a couple of inches apart.  Beat together the egg and salt for the glaze. With a pastry brush, gently brush the glaze on the tops of the dough. 

Bake for 15 minutes and then reduce the oven temperature to 350 degrees F (177 degrees C). Bake for a further 30 to 40 minutes or until the shells are a nice amber color and when split, are dry inside. Turn the oven off and, with the oven door slightly ajar, let the shells dry out for a further 10 - 15 minutes. Remove from oven and let cool on a wire rack.

For whipped cream:  In a large mixing bowl place the whipping cream, vanilla extract, and sugar and stir to combine. Cover and chill the bowl and whisk in the refrigerator for at least 30 minutes. When chilled, whip the mixture until soft peaks form. 

To Assemble:  Split the pastry shells in half and fill (or pipe) with whipped cream. Place the top half of the pastry shell on the whipped cream and dust with powdered (confectioners or icing) sugar.

Makes 12 cream puffs.