Combine the milk, ½ cup cream, and scraped vanilla seeds in a medium pot and place on medium-high heat whisking occasionally. Bring to a boil and immediately turn off the heat and set aside.
In a large bowl, whisk the sugar, cornstarch, and salt together then add the egg yolks. Whisk until pale yellow and smooth. While whisking, slowly pour in ½ cup of the hot milk mixture until incorporated. Whisk in the remaining hot milk mixture. Pour the mixture through a strainer back into the saucepan.
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.
For the Choux Pastry:
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.)
For the Assembly:
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.
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Notes
Use a scale to measure the dry ingredients. Flour, in particular, is easily mismeasured when using a measuring cup. For a reliable choux, measure the flour by weight. If you don’t have a scale, fluff the flour in its container, spoon it into a measuring cup, and level off the top with a knife.
Use room temperature eggs. Make sure the eggs for the choux pastry come to room temperature so they can blend effortlessly, and you can avoid over-mixing. Place your eggs in a bowl of warm tap water if they are cold.
When adding the eggs to the choux pastry dough, make sure the flour mixture is warm but not piping hot. You do not want the egg yolks to cook from the heat.
The number of eggs you’ll need for the choux pastry varies. The size of the yolks, the humidity of your kitchen, or even a bit of extra flour will affect the number of eggs you need. Hold off on adding the last egg to see if your dough needs it. Only add enough eggs for your dough to be shiny, thick, and smooth.
Pat the points of the piped dough before baking. Gently tapping the tops of the piped dough to pat down the spike prevents the tip from burning and makes a more uniformly shaped puff.
For more color on the choux pastry during baking, brush the tops of the dough lightly with an egg wash before baking. For an egg wash, beat an egg lightly with 1 teaspoon of milk.
Let the choux pastry cool before adding the filling. If you rush the process, the heat will melt the filling.