In a mixing bowl, combine the all-purpose flour, granulated sugar, and softened unsalted butter. Use your fingers or a pastry cutter to blend until resembling coarse crumbs.

Press this mixture evenly into the bottom of a lined 9x9-inch baking pan. Bake in a preheated oven at 350°F for 15-20 minutes, until lightly golden.

In a saucepan over medium heat, warm the milk until steaming but not boiling. Whisk together the eggs, granulated sugar, and cornstarch in a separate bowl until smooth.

Slowly pour the warm milk into the egg mixture while whisking to prevent curdling. Transfer back to saucepan and cook on low heat, stirring constantly for about 5-7 minutes until custard thickens.

Remove from heat and stir in the vanilla extract. Allow custard to cool slightly for about 10 minutes.

Pour the warm custard over the baked crust. Spread evenly with a spatula.

Cover with plastic wrap and place in the refrigerator to chill for at least 4 hours, until custard is set.

Once chilled, use a warm knife to cut into squares. Serve chilled, optionally dusted with powdered sugar or garnished with fresh berries.