In a mixing bowl, combine softened butter, powdered sugar, and all-purpose flour until the mixture resembles coarse crumbs. Add an egg, mixing until the dough comes together in a smooth ball. Wrap the dough in plastic wrap and chill in the refrigerator for at least 30 minutes.

Preheat your oven to 375°F (190°C). Roll out the chilled dough on a floured surface to fit your tart pan, press it into the pan, and bake for 20 minutes until golden. Let it cool on a wire rack.

In a heatproof bowl, whisk together eggs and sugar until pale. Gradually add warmed milk mixed with vanilla, whisking constantly. Cook over medium heat until thickened (5-7 minutes), remove from heat, and cool completely.

Once the crust is cooled, fill it with the pastry cream, smoothing the top. Chill for at least 1 hour to set.

Before serving, decorate the tart with fresh berries, powdered sugar, and optional almond slivers or mint leaves. Serve chilled.