Cook the egg yolk mixture over medium-high heat, whisking constantly, until thickened and just starting to bubble. The whisk should leave a defined trace when moved through the custard. Remove from the heat and whisk in the butter until fully melted and combined. Transfer the custard to a large bowl.

Cover with plastic wrap, lightly pressing the plastic against the surface to prevent a skin from forming. Chill for at least 2 hours or until ready to serve. The custard can be made up to 2 days in advance.

Preheat the oven to 425F. Line a large sheet pan with parchment paper.

In a large saucepan, bring the water, butter, sugar, and salt to a boil over medium-high heat. When it boils, immediately take the pan off the heat.

Add all of the flour at once and stir hard with a wooden spoon until all of the flour is incorporated. Return to medium heat and cook, stirring for about 60 seconds.

Scrape the mixture into the bowl of a stand mixer fitted with a paddle attachment (you can also use a hand mixer). Beat at medium speed for about 2 minutes to cool the mixture down. (it will still be warm.)

With the mixer running, add 3 eggs, one at a time, stopping to scrape down the sides of the bowl between each egg. Mix until the dough is smooth and glossy and the eggs are completely incorporated. The dough should be thick but fall slowly and steadily from the beater when lifted out of the bowl. If the dough is still clinging to the beater, add the remaining 1 egg and mix until incorporated.

Transfer the dough to a pastry bag fitted with a large round tip and pipe dollops roughly 2 inches in diameter and 1 inch high. Lightly wet a finger and tap down the spike on top. (For additional color during baking, you can brush the tops of the dough lightly with an egg wash.)

Bake for about 20 minutes or until the dough is golden brown and appears dry. (Try not to open the oven door during the bake.) Remove the baking sheet from the oven. Carefully poke holes in the side of each puff by inserting the tip of a small knife about halfway into the puff. Return to the oven and bake for another 3 minutes. Remove and let cool completely on the baking sheet. (You can turn the oven off and leave the puffs in the oven with the door cracked open after baking if you'd like them to really dry out.)

Place the remaining 1 cup cold cream from the filling in a large bowl and beat until stiff peaks form. Remove the custard from the fridge and whisk it to break it up and smooth the texture out. Fold the whipped cream into the custard. Transfer the filling to a piping bag fitted with a star tip

Cut the tops of the cooled puffs off with a serrated knife. You can scoop out some of the softer strands inside, if desired. Pipe the cream filling into each pastry then place the cap on, and lightly dust with confectioners’ sugar. Cream puffs are best if enjoyed within a few hours of assembly. Any leftovers can be stored in the fridge in an airtight container for up to 2 days.