Preheat your oven to 350°F (175°C). Line an 8x8 inch baking pan with parchment paper, leaving some overhang.
In a large mixing bowl, combine the flour, sugar, and salt.
Grate the cold butter into the flour mixture using a box grater. Gently mix the grated butter into the flour mixture with your hands or a fork until it resembles coarse crumbs.
Set aside about 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, dump in the blueberries, maple syrup, lemon juice, and cornstarch.
Cook the mixture over medium heat, stirring constantly, until it 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 extra 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, carefully lift out of the pan using the parchment overhang. Cut into squares or bars and serve.