Preheat your oven to 350°F (175°C). Line an 8x8 inch baking pan with parchment paper, leaving some overhang for easy removal.

In a large mixing bowl, combine the flour, sugar, and salt. Grate the cold butter into the flour mixture using a box grater. Mix the grated butter into the flour mixture with your hands or a fork until it resembles coarse crumbs. Set aside ¾ cup of the mixture for the crumble topping.

Press the remaining mixture firmly and evenly into the bottom of the prepared baking pan.

In a medium saucepan, combine the blueberries, maple syrup, lemon juice, and cornstarch. Cook over medium heat, stirring constantly, until the mixture thickens and the blueberries begin to burst, about 5-7 minutes. Remove from heat and stir in the vanilla extract. Let cool slightly.

Spread the blueberry filling evenly over the crust in the baking pan. Sprinkle the reserved crumble mixture evenly over the blueberry filling. Bake in the preheated oven for 35-40 minutes, or until the crust and crumble topping are golden brown.

Remove from the oven and let cool completely in the pan on a wire rack. Once cooled, lift the bars out of the pan using the parchment paper overhang. Cut into squares or bars and serve.