1. Prepare the Shortbread Crust:

1. Preheat oven to 350°F (175°C).
2. In a food processor, combine flour, sugar, and salt. Add cold butter and pulse until the mixture resembles coarse crumbs.
3. Add the egg yolk and pulse again. If the dough is too dry, add ice water 1 tablespoon at a time, just until it comes together.
4. Press the dough evenly into the bottom of a 9×9-inch square baking pan, lined with parchment paper or greased.
5. Prick the dough lightly with a fork to prevent bubbling.
6. Bake for 15-18 minutes, until golden brown. Let cool slightly while preparing the custard.

2. Make the Custard Filling:

1. In a saucepan, heat milk and heavy cream over medium heat until it just begins to simmer (do not boil).
2. In a separate bowl, whisk egg yolks, sugar, cornstarch, and salt until smooth.
3. Slowly pour a small amount of hot milk into the egg mixture, whisking constantly to temper the eggs. Gradually add the rest while whisking.
4. Pour the mixture back into the saucepan and cook over medium heat, whisking constantly, for 5-7 minutes until thickened.
5. Remove from heat and stir in vanilla extract.

3. Assemble the Custard Squares:

1. Pour the hot custard over the slightly cooled shortbread crust. Smooth the top with a spatula.
2. Bake at 350°F (175°C) for 20-25 minutes, until the custard is set but still slightly jiggly in the center.
3. Let cool to room temperature, then refrigerate for at least 2-3 hours (or overnight) until fully set.

4. Finish and Serve:

1. Dust the chilled custard squares with powdered sugar.
2. Slice into squares and serve chilled for the best texture and flavor.
3. Enjoy these buttery, creamy, and lightly sweet custard squares at any gathering!