Make the custard: In a medium microwave-safe bowl, whisk together the egg yolks, sugar, cornstarch, vanilla, and milk. Cover with plastic wrap.
Cook the custard in a microwave for 4 minutes, stirring after every 2 minutes until very smooth. Re-cover with plastic wrap, pressing onto the surface of the cream to prevent a skin from forming. Chill in the fridge until cool, about 30 minutes.
Preheat the oven to 375˚F (190˚C). Line a baking sheet with parchment paper.
Make the pastry: In a medium saucepan, bring the water, butter, and salt to a boil over medium heat.
As soon as the mixture begins to boil, reduce the heat to medium-low and add the flour. Using a wooden spoon, mix thoroughly to combine.
Continue stirring (about two minutes). The dough should form into a ball and easily pull away from the sides of the pan. Remove the mixture from the heat and let cool for 5 minutes.
Add 4 of the eggs to the cooled dough, one at a time, being sure to fully incorporate each egg before adding the next. (The eggs will initially be difficult to incorporate into the dough, but continue stirring vigorously and they will eventually come together to form a smooth, shiny dough. When the dough drops making a V shape, it is ready!)
Transfer the dough to a piping bag and pipe onto the baking sheet in an 7-inch (17-cm) ring shape. Pipe a second ring outside the first ring. Pipe a final ring on top. Beat the remaining egg, then brush over the ring.
Bake for 20 minutes.
Then, without opening the oven, reduce the heat to 350˚F(180˚C) and bake for another 15 minutes.
Turn off the oven and leave the puff pastry inside for another 10 minutes.
Cut the choux pastry in half horizontally. Pipe the custard into the hollow center of the shell. Pipe whipped cream on top of the custard. Top with berries. Place the top half of the shell. Dust with powdered sugar.