Prepare the Oven & Pan:
Preheat your oven to 350°F (175°C). Line a 9x13-inch baking dish with parchment paper for easy removal of the bars.

Make the Crust:
In a medium bowl, whisk together 2 1/2 cups flour, powdered sugar, and salt. Add the melted butter and mix until fully combined. For extra flavor, add a splash of vanilla extract with the butter.
Press the dough evenly into the prepared baking dish to form the crust. Bake at 350°F (175°C) for about 20 minutes, or until the crust is lightly golden.

Prepare the Lemon Blueberry Filling:
While the crust is baking, place the sugar and lemon zest in a food processor. Pulse until the zest is fully incorporated and the sugar turns light yellow. If you don’t have a processor, finely mince the zest and mix it with the sugar in a bowl.
In a large bowl, combine the zesty sugar with 1/2 cup flour and whisk until smooth.
Add the fresh lemon juice and eggs and mix until completely combined.
Gently fold in the blueberries, making sure not to break them up too much (if using frozen, fold in gently while still frozen to prevent bleeding).

Make the Crumble Topping:
In a large bowl, mix together the vanilla cake mix and the melted butter until crumbles form. coarse crumbs. You want some larger chunks of butter for that perfect crumble texture!

Assemble & Bake:
Once the crust is baked and golden, pour the lemon-blueberry filling over the crust. Then, sprinkle the crumble topping evenly over the filling.
Return the pan to the oven and bake for 30-35 minutes, or until the filling is set and the crumble is golden and crispy. You may want to rotate the pan halfway through for even baking.

Cool & Chill:
After baking, let the bars cool in the pan for about 1 hour. Then, transfer to the refrigerator and chill for at least 2 hours to set the filling and allow the crumble to firm up.

Serve:
Once fully chilled, dust with powdered sugar for a finishing touch. Slice into bars using a sharp, damp knife (wipe the knife between cuts for clean edges).