To make the pastry:

1. Preheat the oven to 400 degrees F. Line a baking sheet with parchment paper or a baking mat and set aside.
2. Add the water and butter to medium sauce pan and bring to a boil over medium-high heat.
3. Once boiling turn heat to low and add flour all at once; stir vigorously for 1 minute.
4. Remove from heat. Let the mixture cool for 5 minutes.
5. Add eggs one at a time, and beat very well between each addition. The mixture will go from slimy and wet looking to smooth and uniform after each addition. This takes a little elbow grease or you can use a stand mixer if you prefer. 
6. Put the warm dough in a large piping bag (I don't use a tip, just the end of the open bag, you can use a large plain round tip if you'd like). 
7. Pipe into mounds on parchment lined cookie sheet about 2 ½ inches wide and tall.
8. Bake 35-40 minutes until puffed and golden brown. Remove from the oven and allow to cool to room temperature (about an hour) before filling. 

To make the pastry cream filling:

1. In a large saucepan, bring the milk and 1/4 cup of sugar up to 180 degrees over medium heat, stirring very often. The mixture should be hot and start to steam across the top, but should not boil.
2. While the milk is heating, mix the cornstarch and 1/4 cup of sugar together in a medium bowl.
3. Add the egg yolks and whole egg to the cornstarch mixture and whisk to combine well.
4. When the milk is hot, slowly whisk one cup of hot milk into the egg mixture and then whisk the egg mixture back into the pan of hot milk.
5. Cook over medium heat until the mixture starts to bubble and thicken, stirring constantly, about 6 minutes.
6. Remove from the heat and stir in butter, and vanilla.
7. Let filling cool completely before filling pastry.

To assemble:

1. Cut off top of the pastry.
2. Fill the bottom of the pastry up with the pastry cream filling. Fill the top with the whipped cream. 
3.  Put the top on bottom and repeat with remaining pastries, pastry cream, and whipped cream.
4. Sprinkle the tops with powdered sugar and serve!