Line a baking sheet with parchment paper. Preheat the oven to 405º F.
In a saucepan over medium heat, mix together the butter, water, sugar, and salt. Bring the mixture to a boil.
Add the flour and beat the mixture with a wooden spoon for 2 minutes over the heat. Remove the dough from the heat and place it in a mixing bowl. Beat the dough for 3 more minutes.
Mix in the eggs, one at a time until the mixture is smooth. Transfer the pastry to a piping bag with a 1/2-inch round tip.
Pipe the pastry into small circles of your desired size onto the prepared baking sheet. Gently smooth ou the tips of the pastry with wetted fingers. Brush the cream puffs with the beaten egg.
Bake the puffs at 405º for 20 minutes. Lower the oven temperature to 300º without opening the oven and bake the puffs for 20 more minutes. Turn the oven off and let the pastry dry in the oven for 30 more minutes.
Remove the puffs from the oven and let them cool. Poke a hole in the bottom of each puff using a 1/4-inch tip of a piping bag.
In a mixing bowl, whip the heavy cream until stiff peaks form. Fold in the chilled pastry cream until everything is combined. Transfer the pastry cream to a piping bag with a 1/4-inch piping tip.
Pipe the cream into the puffs until they are filled and feel heavy. Let the cream puffs chill until ready to serve.