Preheat your oven to 350°F (175°C). In a mixing bowl, combine the unsalted butter, all-purpose flour, powdered sugar, and a pinch of salt until crumbly. Press this mixture evenly into the bottom of a greased 8x8-inch baking dish. Bake for 15-20 minutes, or until the crust is lightly golden and firm to the touch. Remove from the oven and let cool slightly.

In a saucepan over medium heat, warm 2 cups of whole milk until just simmering, then remove from heat. In a separate bowl, whisk together the egg yolks, granulated sugar, cornstarch, and vanilla extract until smooth. Gradually whisk about 1/4 cup of the warm milk into the egg mixture to temper the eggs, preventing curdling, then whisk this mixture back into the saucepan with the remaining milk.

Return the saucepan to medium heat, stirring continuously until the mixture thickens and starts to bubble, about 5-7 minutes. Once thickened, pour the custard over the pre-baked crust, smoothing it out with a spatula.

Place the baking dish back in the oven and bake for an additional 25-30 minutes. Check for doneness by gently shaking the pan; the custard should be set but slightly jiggly in the center.

Once baked, remove the dish from the oven and let it cool at room temperature for about 30 minutes. Cover with plastic wrap and refrigerate for at least 3 hours or overnight for best results.

Before serving, dust the top of the chilled Vanilla Custard Cream Squares with powdered sugar. Cut into squares and serve with optional garnishes such as fresh berries or whipped cream.