In a saucepan, heat the water, butter, sugar, and salt until it reaches a rolling boil. Remove from heat, add the flour all at once, and stir vigorously until the mixture forms a smooth dough ball.

Add the eggs one at a time, mixing well after each addition until fully incorporated. The dough should be smooth and glossy. Transfer to a piping bag fitted with a round or star tip.

Preheat the oven to 400°F (200°C). Line a baking sheet with parchment paper. Pipe 20 evenly sized mounds of dough, leaving space between them. Bake for 20 minutes at 400°F, then reduce the temperature to 350°F (180°C) and bake for another 20 minutes until golden brown. Let them cool completely before filling.

Gradually pour the hot milk into the egg mixture while whisking. Return everything to the saucepan and cook over low heat, stirring continuously, until thickened.

Transfer the cream to a bowl, cover with plastic wrap touching the surface, and let it cool completely. Once cooled, beat in the softened butter until smooth and creamy.

In a large bowl, beat the heavy cream until stiff peaks form. Optionally, add powdered sugar for extra sweetness.

Slice each cooled pastry in half. Pipe a generous layer of pastry cream onto the bottom half, followed by a layer of whipped cream. Place the top half of the pastry over the filling.