Preheat the oven to 350°F (175°C). Grease an 8x8-inch baking dish or line it with parchment paper.

In a mixing bowl, combine the flour, softened butter, 1/4 cup granulated sugar, and salt. Mix until crumbly. Add the egg yolk and mix until the dough forms.

Press the dough evenly into the bottom of the prepared baking dish. Bake for 15-20 minutes, or until lightly golden. Remove from the oven and let it cool.

In a medium saucepan, whisk together 2 cups of milk, 1/2 cup granulated sugar, cornstarch, and 1/4 teaspoon salt over medium heat. Cook, stirring constantly, until the mixture thickens and starts to bubble.

In a separate bowl, whisk the 4 egg yolks. Gradually add a small amount of the hot milk mixture to the egg yolks, whisking constantly to temper the eggs.

Pour the tempered egg mixture back into the saucepan with the remaining milk mixture. Cook for an additional 2-3 minutes, stirring constantly until thickened. Remove from heat and stir in the vanilla extract.

Pour the custard over the cooled crust in the baking dish, spreading it evenly. Allow it to cool to room temperature, then refrigerate for at least 2 hours to set.

Before serving, whip the heavy cream until soft peaks form. Spread or pipe the whipped cream over the custard layer. Dust with powdered sugar before slicing into squares.