Preheat the oven to 350℉. Grease a 9x13-inch baking dish with butter or non-stick cooking spray. Set aside.
Make the Filling
In a 4-quart (or larger) nonstick sauce pan set over medium-high heat, stir together blueberries, sugar, cornstarch, cinnamon, vanilla extract and salt. Simmer for 20 minutes, until the berries begin to burst. Remove from heat and set aside to cool while you make the crust.
Make the Crust
Using a sharp knife, cut the butter into cubes. Using a fork, pastry blender, or the whisk attachment on an electric mixer, combine butter, flour, and sugar until a crumbly dough forms.
Press the crust mixture into an even layer in the bottom of the greased pan. Bake in preheated oven for about 15 minutes, until crust is golden brown.While the crust is baking, make the crumble topping.
For the topping: In a medium sized bowl, stir melted butter, sugar, cinnamon and flour together.
Remove crust from the oven. Pour blueberry filling into the crust and use an offset spatula or spoon to spread it into an even layer. Sprinkle streusel topping over the filling.
Bake in preheated oven for 15-20 minutes, until streusel is golden brown. Remove dessert from the oven and transfer to a wire rack to cool before cutting into squares.
Notes
Canned or frozen blueberries can be substituted for fresh blueberries in this recipe. There is no need to thaw frozen blueberries first, as the filling gets cooked anyway.
Refrigerate the bars in a covered container and use within 5 days.
The dessert does not freeze well, but the pie filling can be frozen. Store in a freezer-safe container for up to 3 months.