Step 1: Prepare the Vanilla Custard

1. In a medium pot, combine the milk, ½ cup of heavy cream, and vanilla seeds. Heat over medium-high heat, whisking occasionally, until boiling. Remove from heat.
2. In a separate bowl, whisk together the sugar, cornstarch, and salt. Add the egg yolks and continue whisking until pale yellow and smooth.
3. Gradually pour half of the hot milk mixture into the egg yolk mixture while whisking continuously. Then add the remaining hot milk mixture and strain the combined liquid back into the pot.
4. Return the pot to medium-high heat and cook, whisking constantly, until the custard thickens and leaves a defined trail when stirred. Remove from heat and whisk in the butter until fully incorporated.
5. Transfer the custard to a bowl, cover with plastic wrap pressed against the surface, and chill for at least 2 hours or up to 2 days.

Step 2: Make the Choux Pastry

1. Preheat your oven to 425°F (220°C). Line a baking sheet with parchment paper.
2. In a saucepan, bring the water, butter, sugar, and salt to a boil over medium-high heat. Once boiling, remove from heat and immediately add all the flour at once. Stir vigorously until fully combined.
3. Return the pan to low heat and cook for about 1 minute, stirring constantly to dry out the dough slightly. Transfer the dough to a stand mixer fitted with a paddle attachment (or use a hand mixer).
4. Beat the dough on medium speed for 2 minutes to cool it slightly. With the mixer running, add 3 eggs one at a time, stopping to scrape down the sides between each addition. Mix until the dough is smooth, glossy, and falls slowly from the beater. If needed, add the fourth egg.
5. Transfer the dough to a piping bag fitted with a large round or star tip. Pipe mounds roughly 2 inches wide and 1 inch high onto the prepared baking sheet. Use a damp finger to pat down any spikes.
6. Bake for 20 minutes or until golden brown. Carefully poke holes in the side of each puff with a small knife and return to the oven for an additional 3 minutes. Let cool completely.

Step 3: Assemble the Cream Puffs

1. Whip the remaining 1 cup of heavy cream until stiff peaks form. Remove the chilled custard from the fridge and whisk briefly to loosen its texture. Fold the whipped cream into the custard until no streaks remain.
2. Transfer the filling to a piping bag fitted with a star tip. Cut off the tops of the cooled puffs and pipe the filling inside. Replace the tops and lightly dust with confectioner’s sugar.