Preheat oven to 350 degrees °F.

Add 3 sticks of butter to a skillet or pan and brown over low heat. Reserve one cup of the brown butter to the shortbread crust and reserve the remaining amount for the crumble topping.

Start by making the crumble topping. In a bowl, add 1/2 cup of browned butter, all purpose flour, brown sugar, and kosher salt. Toss together with your fingers until clumps begin to form. Add to the freezer until ready to use.

Now let's make the crust. Add 1 cup of the brown butter to a large bowl. Add brown sugar, kosher salt, vanilla extract, and all purpose flour. Mix together until a dough forms. Line an 8x8 baking dish with parchment paper and place dough onto the bottom, pressing it firmly into each corner and ensuring it is spread evenly throughout the dish. Bake for 20 minutes.

Add raspberry preserves, blueberries, sliced strawberries, blackberries, and 1/2 lemon squeezed into a sauce pan over medium low heat. Bring to a boil and simmer for 15 minutes. In a small bowl, add corn starch and a few tablespoons of the berry mixture and stir together to form a thick paste. Add to the berry sauce and remove from heat to cool slightly and thicken.

Pour berry sauce on top of shortbread crust in the 8x8 baking dish. Remove crumble topping from freezer and generously pour on top of berry sauce, ensuring everything is covered. Bake for 45 minutes.

Once the bars are finished baking, allow them to cool for at least an hour, then refrigerate for 2 hours.