Combine the milk, ½ cup cream, and scraped vanilla seeds in a medium pot and heat on medium-high until boiling. Remove from heat and set aside.

In a large bowl, whisk together sugar, cornstarch, and salt. Add egg yolks and whisk until pale yellow and smooth. Slowly pour in ½ cup of the hot milk mixture while whisking, then whisk in the remaining hot milk mixture. Strain back into the saucepan.

Cook the egg yolk mixture over medium-high heat, whisking constantly, until thickened and starting to bubble. Remove from heat and whisk in butter until melted. Transfer to a large bowl and cover with plastic wrap, pressing it against the surface. Chill for at least 2 hours.

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

In a large saucepan, bring water, butter, sugar, and salt to a boil over medium-high heat. Remove from heat and add flour all at once, stirring until combined. Return to medium heat and cook, stirring for about 60 seconds.

Transfer to a stand mixer bowl and beat at medium speed for about 2 minutes to cool. Add 3 eggs, one at a time, mixing until smooth and glossy. If needed, add the remaining egg.

Pipe dollops of dough roughly 2 inches in diameter onto the prepared baking sheet. Tap down the spikes and optionally brush with egg wash. Bake for about 20 minutes until golden brown, then poke holes in the side of each puff and bake for another 3 minutes. Let cool completely on the baking sheet.

In a large bowl, beat the remaining 1 cup cold cream until stiff peaks form. Whisk the chilled custard to smooth it out and fold in the whipped cream. Transfer the filling to a piping bag fitted with a star tip.

Cut the tops off the cooled puffs, pipe in the cream filling, replace the tops, and dust with confectioner's sugar. Enjoy within a few hours or store in the fridge for up to 2 days.