Place 1 ½ cups of milk in a small pot over low heat and add the vanilla bean and seeds.

Place ½ cup of sugar, ¼ cup of cornstarch, and ½ teaspoon of salt in a large bowl and whisk to combine.

Stir in the 6 egg yolks until smooth.

When the milk is steamy, start whisking a ladleful at a time into the yolk mixture to slowly warm it without scrambling the eggs.

Once all the milk has been added, transfer the mixture back into the pot and cook over medium-low heat (whisking constantly) until it reaches a temperature of approximately 185 degrees and becomes very thick.

Remove from the heat and immediately pour through a fine mesh strainer into a heat-safe bowl (use the back of a ladle to push it through).

Press plastic wrap directly onto the surface and chill until cold and firm (approx. 2 hours).

Place 1 cup of softened butter in a large mixing bowl with 3 tablespoons of granulated sugar.

Using the whip attachment, cream 1 cup of soft unsalted butter and 3 tablespoons of granulated sugar together until very pale and fluffy (approx. 5 minutes on medium-high speed).

Scrape the bottom and sides of the bowl with a silicone spatula and add the chilled pastry cream in a spoonful at a time, whipping after each addition.

Preheat the oven to 425 degrees F and grease and line (2) 8-inch diameter cake pans with circles cut from parchment paper.

Place the water, butter, sugar, and salt in a small pot and cook over medium-high heat until the butter is completely melted and the mixture is simmering.

Add in the flour all at once, and continue to cook, whisking, for about 5 minutes, or until the dough gathers itself into a ball and a film begins to form on the bottom of the pot.

Transfer the mixture to a mixing bowl, and beat on medium speed.

Drop in the eggs, one at a time, while continuing to beat. (Allow each egg to become fully incorporated before adding the next.)

Divide the dough equally between the two cake pans, smoothing one out into an even layer and swirling the other to create peaks and valleys.

Bake until puffed, golden brown, hollow, dry and light (approx. 45 to 55 minutes).

Cool completely, then spread the German buttercream filling over the flatter cake layer, and top with the remaining cake layer.

Dust with powdered sugar, for garnish.