Prepare a 9 x 13 baking dish with cooking spray and foil or parchment. This will make the bars easier to remove from the pan.

Line a 13×9-inch metal baking pan with foil, leaving an overhang on each side so you can lift the bars out after baking and cooling. Spray the pan and foil with nonstick cooking spray.

Browned Butter

Add the butter to a medium saucepan set over medium-low heat. Melt the butter, swirling the pan frequently. Once the butter has melted, it will start to bubble and foam. Continue to cook (continuing to swirl the pan) until the butter turns brown and smells nutty. Be patient and it watch carefully, it can go from brown to burned quickly. Turn off the heat under the pan, and let the butter cool until lukewarm.

Blueberry Filling

In a saucepan combine 3 cups of blueberries, sugar, cornstarch, lemon juice and salt. Cook over medium heat, stirring frequently until the blueberries start to release their juices. About 5-7 minutes.

Increase the heat to medium-high and bring to a boil, stir the mixture constantly so it does not burn. Boil for 1 minute and remove from the heat. Add the remaining 2 cups of berries and the vanilla and mix.  Cool the mixture while you make the crust and crumb mixture.

Crust/Crumb Topping

In a medium bowl, whisk together the cooled brown butter, 3/4 cup of the sugar, and the salt. Beat in the egg yolks. Use a rubber spatula to stir in the flour until completely incorporated. The dough will be thick and stiff. Transfer about 2 cups to the prepared pan and press into an even layer in the bottom of the pan. Prick the dough all over with a fork, then chill in the refrigerator for at least 30 minutes, or until firm.

Meanwhile, preheat oven to 325 F.

Bake the crust for about 20 minutes, or until it begins to set, but has not yet started to brown around the edges.

Crumb Topping

Combine the remaining 1/4 cup of sugar with the reserved dough. Use your fingers to mix until crumbly.

Spread the blueberry mixture in an even layer over the hot crust. Sprinkle the streusel over the top. Increase the oven temperature to 350 F, and bake on a rack placed in the top of the oven for about 25 minutes, or until the streusel is golden.

Transfer the pan to a wire rack and cool for at least 1 hour. Once cool, use the foil handles to lift the bars out. Place on a cutting board, and cut into squares before serving. Store for up to a week in an airtight container at room temperature.