• In a medium saucepan, combine the milk, ½ cup cream, and vanilla seeds. Warm over medium-high heat, whisking occasionally, until it just begins to boil. Remove from heat and set aside.
• In a large bowl, whisk together sugar, cornstarch, salt, and egg yolks until smooth and pale yellow. Slowly pour in ½ cup of the warm milk mixture while whisking to temper the eggs, then whisk in the rest. Strain the mixture back into the saucepan.
• Cook over medium-high heat, whisking constantly, until the custard thickens and bubbles slightly. Remove from heat and whisk in the butter until fully melted and incorporated.
• Pour the custard into a bowl and cover with plastic wrap, pressing it directly onto the surface to prevent a skin from forming. Chill for at least 2 hours (or up to 2 days) before serving.

For the Choux Pastry:

• Preheat your oven to 425°F (220°C) and line a large baking sheet with parchment paper.
• In a large saucepan, combine water, butter, sugar, and salt. Bring to a boil over medium-high heat. As soon as it boils, remove the pan from the heat.
• Add all the flour at once and stir vigorously with a wooden spoon until the mixture forms a smooth dough. Return the pan to medium heat and cook, stirring constantly, for about 1 minute to cook out the raw flour taste.
• Transfer the dough to the bowl of a stand mixer fitted with a paddle attachment (or use a hand mixer). Beat on medium speed for 2 minutes to cool the dough slightly—it should still be warm but not hot.
• With the mixer running, add 3 eggs, one at a time, ensuring each is fully incorporated before adding the next. Scrape down the sides of the bowl as needed. Mix until the dough is glossy, smooth, and thick enough to slowly fall from the beater in a steady ribbon. If the dough seems too stiff, add the remaining egg and mix until combined.
• Transfer the dough to a piping bag fitted with a large round tip. Pipe 2-inch-wide, 1-inch-high mounds onto the prepared baking sheet. Smooth any peaks with a lightly wet finger. For a golden finish, brush the tops lightly with egg wash.
• Bake for 20 minutes, or until the puffs are golden brown and dry to the touch. Avoid opening the oven during baking. Once done, remove the sheet and carefully poke a small hole in the side of each puff with a knife to release steam. Return to the oven for 3 more minutes to dry out the centers.
• Let the puffs cool completely on the baking sheet. For extra crispness, you can turn off the oven and leave the puffs inside with the door slightly ajar for a few minutes.

For the Assembly:

• In a large bowl, whip the remaining 1 cup of cold cream until it forms stiff peaks. Take the chilled custard from the refrigerator and give it a quick whisk to smooth out the texture. Gently fold the whipped cream into the custard until fully combined. Transfer the lightened filling to a piping bag fitted with a star tip for a beautiful finish.
• Using a serrated knife, carefully slice the tops off the cooled choux puffs. If desired, remove any soft strands inside to create more space for the filling. Pipe the creamy custard mixture generously into each puff, then place the tops back on like little hats.
• For a final touch, dust the assembled cream puffs lightly with confectioners’ sugar. These delicate pastries are best enjoyed within a few hours of assembly, but any leftovers can be stored in an airtight container in the fridge for up to 2 days.s.