1. Line a 9×13-inch baking pan with parchment paper, leaving an overhang on the long sides for easy lifting, and lightly grease the parchment and any exposed pan edges. Preheat the oven to 350°F (175°C).

2. In a mixing bowl, stir together the melted butter, 1/2 cup granulated sugar, 2 cups flour, and salt until a soft, sandy dough forms.

3. Press the dough evenly into the bottom of the prepared pan, making sure to reach all the corners and keep the layer level.

4. Bake the crust for 15–18 minutes, or until the edges are just turning light golden and the surface looks set.

5. While the crust bakes, whisk the eggs and 1 cup granulated sugar in a medium bowl until slightly thickened and pale.

6. Add the sour cream, 3 tablespoons flour, lemon zest, lemon juice, and vanilla to the egg mixture and whisk until completely smooth.

7. When the crust comes out of the oven, pour the lemon filling over the hot crust and gently spread it into an even layer.

8. Return the pan to the oven and bake for 15–18 minutes, or until the lemon layer looks set around the edges but still has a slight wobble in the center.

9. Remove from the oven and place the pan on a wire rack to cool while you prepare the blueberry topping.

10. In a small saucepan, combine the blueberries, 1/3 cup granulated sugar, cornstarch, water, and 1 tablespoon lemon juice.

11. Cook over medium heat, stirring frequently, until the berries begin to burst and the juices thicken into a glossy sauce that coats the back of a spoon, 5–8 minutes.

12. Remove the blueberry mixture from the heat and let it cool for about 10 minutes, until warm but not piping hot.

13. Spoon the warm blueberry topping over the slightly cooled lemon layer, spreading it gently all the way to the edges.

14. Allow the pan to cool completely at room temperature, then transfer it to the refrigerator to chill for at least 2 hours, or until fully cold and set.

15. Use the parchment overhang to lift the chilled slab out of the pan, dust generously with powdered sugar, garnish with lemon slices if desired, and cut into 12–16 bars before serving.