In a large mixing bowl, combine softened unsalted butter, brown sugar, and all-purpose flour until the mixture resembles coarse crumbs, about 3-4 minutes. Add in one large egg and mix until the dough comes together. Shape into a disc, wrap in plastic wrap, and refrigerate for at least 30 minutes to firm up the crust.

Preheat your oven to 375°F (190°C). Roll out the chilled tart dough on a floured surface to about 1/8-inch thickness, then carefully transfer it to a tart pan, pressing it into the edges. Prick the bottom with a fork and place parchment paper over it. Fill with pie weights and bake for 20-25 minutes, or until the crust is golden brown. Remove and allow to cool completely.

In a medium bowl, whisk together 4 egg yolks, 1/2 cup of sugar, and 2 tablespoons of cornstarch until smooth. In a saucepan, heat 2 cups of whole milk with the scraped seeds from 2 vanilla beans over medium heat until it simmers. Gradually whisk the warm milk into the egg mixture, then return to the saucepan. Cook over low heat, stirring constantly for about 5-7 minutes until thickened.

Once the pastry cream has thickened, transfer it to a clean bowl and cover the surface with plastic wrap to prevent a skin from forming. Refrigerate the cream for at least 2 hours, or until fully chilled and set.

Once the tart shell and pastry cream are both completely cooled, gently spoon the chilled cream into the tart shell, smoothing the top with a spatula for an even finish. Place the assembled tart back into the refrigerator to set for an additional 30 minutes.

Before serving your Decadent Tart with Silky Vanilla Bean Pastry Cream, decorate the top with a scattering of fresh seasonal berries, a dusting of powdered sugar, and a few mint leaves. Slice and serve chilled.