To prepare the cream puffs:

1. Preheat oven to 375 degrees F.
2. Line a baking sheet with parchment paper. Set aside.
3. In a large heavy pot over medium heat, combine the water, butter slices and salt. Stir until butter is completely melted. Bring water to a slow boil.
4. Using a wooden spoon, quickly stir in the flour until a soft ball forms. Remove the dough from heat, and allow to rest for 5 minutes.
5. Mix in eggs one at a time until each is fully combined. Gently work the dough until it is sticky but smooth.
6. Drop 1/4 cup portions of the puff dough onto the prepared baking sheet at least 2 inches apart from one another.
7. Prepare the egg wash by mixing the egg yolk and the milk. Using a pastry brush, brush the wash over each puff dough. Bake the puffs until they are firm and golden brown, about 35 minutes.
8. Transfer the baked puffs to a wire rack. Using a toothpick, poke each puff to allow steam to release. Cool the puffs until they are firm, about 1 hour.

To prepare the whipped cream filling:

1. In the bowl of a stand mixer fitted with a whip attachment, whisk together the cream, vanilla and sugar.
2. Beat the whipped cream on medium speed until soft peaks form.

To assemble the cream puffs:

1. Cut the top off of each cream puff, and fill with a generous portion of the whipped cream.
2. Replace each of the tops, then dust the cream puffs with confectioners sugar.