Combine the milk, vanilla bean pod and seeds, and half the sugar in a large high-sided saucepot. Stir to combine and bring to a rolling boil over medium-high heat, stirring occasionally until the sugar dissolves.

Set a large bowl on a damp towel to stabilize it. Whisk together the whole egg, egg yolks, cornstarch, and remaining sugar until pale yellow. Line a baking sheet with plastic wrap or parchment paper.

When the milk reaches a full rolling boil, whisk the egg mixture once more, then slowly pour in a small amount of the hot milk, whisking constantly. Gradually add more milk until you have added about half, then pour in the rest and whisk vigorously to combine.

Return the custard base to the saucepot over medium-high heat. Stir constantly with a silicone spatula until it begins to thicken, then switch to a whisk.

Whisk vigorously, including the corners of the pot, until the cream thickens and comes to a boil. Boil for 1 minute.

Scrape the hot pastry cream onto the prepared baking sheet and spread it out. Cover the surface directly with plastic wrap and poke a few holes. Refrigerate until completely cool, at least 1 hour to overnight.

In the bowl of a stand mixer fitted with the whisk attachment, combine the heavy cream, powdered sugar, and vanilla extract. Whip on high speed until soft peaks form. Transfer to a clean bowl.

Add the cold pastry cream to the stand mixer bowl, removing the vanilla bean pod. Fit with the paddle attachment and beat on medium-high speed until smooth and the texture resembles a thick batter.

Fold about one-third of the whipped cream into the pastry cream aggressively to loosen it. Add the remaining whipped cream and fold gently until smooth and combined.

Transfer to a piping bag or bowl. Refrigerate until ready to use.