Preheat your oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment paper, leaving overhang on the long sides for easy lifting. Lightly grease the parchment.

Make the crust/crumble mixture: In a large bowl, whisk the melted butter, granulated sugar, and brown sugar until glossy. Stir in vanilla, almond extract (if using), salt, cinnamon (if using), and lemon zest.

Add the flour to the bowl and mix with a spatula until large crumbles form and no dry flour remains. The mixture should look sandy but hold together when pressed.

Press about two-thirds of the mixture firmly and evenly into the prepared pan to form the base. Use the bottom of a measuring cup to compact it. Set the remaining third aside for the topping.

Make the blueberry filling: In a medium bowl, toss blueberries with sugar, cornstarch, lemon juice, lemon zest, and a pinch of salt until evenly coated.

Spread the blueberry mixture over the crust in an even layer. If using frozen berries, scatter them evenly and pour any remaining sugar-cornstarch mixture over the top.

Crumble the reserved dough over the blueberries, squeezing some of it into larger clumps for texture. Aim for a mix of small and big pieces to create a rustic top.

Bake for 40–50 minutes, until the top is golden and the blueberry filling is bubbling around the edges. If the top is browning too quickly, tent loosely with foil for the last 10 minutes.

Cool completely in the pan on a rack, at least 2 hours. This helps the filling set so the bars slice cleanly. For sharp edges, chill for 30 minutes before cutting.

Lift the bars out using the parchment overhang. Cut into 16–24 squares, depending on the size you prefer. Dust with powdered sugar if you like and serve.