Preheat your oven to 350°F (175°C). In a mixing bowl, combine softened butter, all-purpose flour, and granulated sugar until crumbly. Press this mixture firmly into the bottom of a greased 9x9-inch baking dish. Bake for 15-20 minutes or until the crust turns lightly golden and fragrant. Let it cool.

In a medium saucepan, gently heat 2 cups of whole milk over medium heat until it begins to steam, but do not boil. In a separate bowl, whisk together 4 eggs, ½ cup granulated sugar, and 1 tablespoon of pure vanilla extract until smooth. Gradually add the warm milk to the egg mixture, whisking constantly to prevent curdling.

Pour the custard mixture evenly over the cooled crust. Bake in the preheated oven for 25-30 minutes, watching for slight jiggliness in the center when done.

Once baked, let the squares cool in the dish at room temperature for about 30 minutes. Cover with plastic wrap and refrigerate for at least 2 hours, or until thoroughly chilled and set.

Dust the chilled custard squares lightly with powdered sugar before serving. Cut into squares and enjoy fresh.