In a mixing bowl, combine softened butter, all-purpose flour, and sugar. Mix until the mixture resembles coarse crumbs. Press into the bottom of a greased 9x9-inch baking dish. Bake at 350°F (175°C) for 15-20 minutes, until golden brown.

In a saucepan, heat whole milk over medium heat until warm, do not let it boil. In a bowl, whisk together eggs and sugar, then slowly temper with the warm milk. Add vanilla extract, return to saucepan and cook over medium heat while stirring until thickened, about 5-7 minutes.

Pour the thickened custard over the freshly baked crust, spreading it evenly. Allow to cool at room temperature for 30 minutes then refrigerate for at least 2 hours.

Once chilled, dust the top with powdered sugar, cut into squares and serve cold.