1. Prepare the Shortbread Crust:

• Preheat Oven: Preheat your oven to 350°F (175°C). Lightly grease a 9×9-inch square baking pan or line it with parchment paper, leaving an overhang for easy removal.
• Combine Dry Ingredients: In a food processor, combine the flour, granulated sugar, and salt. Pulse a few times to mix.
• Add Butter: Add the cold cubed butter and pulse until the mixture resembles coarse crumbs.
• Add Egg Yolk: Add the egg yolk and pulse again until the dough just begins to come together.
• Add Ice Water (If Needed): If the dough seems too crumbly, add ice water, 1 tablespoon at a time, pulsing briefly after each addition, until the dough just holds together when squeezed. Do not process until it forms a solid ball. (Alternatively, mix by hand: whisk dry ingredients, cut in butter with pastry blender or fingers, stir in egg yolk and water).
• Press into Pan: Transfer the dough to the prepared baking pan. Press it firmly and evenly into the bottom of the pan.
• Dock the Crust: Use a fork to prick the dough lightly all over. This helps prevent bubbling during baking.
• Bake: Bake the crust for 15-18 minutes, or until it’s lightly golden brown and set.
• Cool Slightly: Remove from the oven and allow the crust to cool slightly on a wire rack while you prepare the custard filling. Keep the oven preheated.

2. Make the Custard Filling:

• Heat Milk and Cream: In a medium saucepan, combine the milk and heavy cream. Heat over medium heat, stirring occasionally, until the mixture just begins to simmer around the edges (small bubbles form). Do not let it come to a full boil.
• Whisk Yolks and Sugar: While the milk mixture is heating, whisk the egg yolks, granulated sugar, cornstarch, and a pinch of salt together in a separate medium bowl until smooth and slightly thickened.
• Temper Eggs (Crucial Step): Once the milk/cream mixture is simmering, slowly pour about half a cup of the hot liquid into the egg yolk mixture while whisking constantly. This tempering process prevents the eggs from scrambling.
• Combine Mixtures: Gradually pour the tempered egg yolk mixture back into the saucepan with the remaining hot milk/cream, whisking continuously.
• Cook Custard: Cook over medium heat, whisking constantly, until the custard thickens enough to coat the back of a spoon (about 5-7 minutes). If you run your finger across the back of the spoon, it should leave a clear trail. Do not let the custard boil vigorously.
• Add Vanilla: Remove the saucepan from the heat and stir in the vanilla extract.

3. Assemble and Bake:

• Pour Custard: Pour the hot custard filling evenly over the slightly cooled shortbread crust in the baking pan. Smooth the top with a spatula.
• Bake: Return the pan to the oven and bake for 20-25 minutes, or until the custard is set around the edges but still has a slight jiggle in the very center.

4. Cool and Chill:

• Cool Completely: Remove the pan from the oven and let the custard squares cool completely to room temperature on a wire rack.
• Chill: Once cooled, cover the pan with plastic wrap and refrigerate for at least 2-3 hours (or preferably overnight) until the custard is fully set and chilled.

5. Finish and Serve:

• Dust (Optional): Once chilled, dust the top of the custard squares generously with powdered sugar, if desired.
• Slice: Use a sharp knife to cut into squares. Wipe the knife clean between cuts for neat slices.
• Serve: Serve chilled.