1. Preheat the oven to 350°F (180°C). Line a 9×13 inch baking pan with parchment paper, leaving an overhang on two sides, and lightly grease the parchment and exposed pan edges.

2. Make the crust by whisking together the flour, granulated sugar, and salt in a mixing bowl, then stirring in the melted butter until the mixture looks like damp sand and starts to clump.

3. Press the crust mixture firmly and evenly into the prepared pan using your fingertips or the bottom of a measuring cup.

4. Bake the crust for about 15 minutes, or until the edges are lightly golden and the surface looks set but not browned.

5. While the crust bakes, prepare the blueberry layer by tossing the blueberries with the sugar, cornstarch, and lemon juice in a small bowl until evenly coated.

6. In a separate bowl, make the lemon filling by whisking together the granulated sugar and flour, then whisking in the eggs one at a time until smooth.

7. Add the lemon juice, lemon zest, and salt to the filling and whisk until the mixture is silky and fully combined.

8. When the crust is finished baking, remove the pan from the oven and immediately scatter the blueberry mixture evenly over the hot crust.

9. Slowly pour the lemon filling over the blueberries, tilting the pan gently so the custard spreads into an even layer without displacing the berries too much.

10. Return the pan to the oven and bake for 20 to 25 minutes, or until the center is set, the edges are lightly golden, and the lemon layer no longer jiggles when you gently shake the pan.

11. Transfer the pan to a wire rack and let the bars cool completely at room temperature.

12. Once cool, refrigerate the pan for at least 2 hours so the lemon layer can firm up for cleaner slicing.

13. Use the parchment overhang to lift the chilled slab of bars onto a cutting board, then dust generously with powdered sugar.

14. Slice into 12 to 16 bars, wiping the knife clean between cuts, and serve chilled or slightly cool.