Preheat oven to 350F (175C) and lightly grease a 9x13 glass baking dish. Set aside.
In small saucepan, combine 3 cups blueberries (505g), sugar, cornstarch, salt, and lemon juice over medium-low heat.
Stir frequently until the blueberries release their juices, and the mixture is very liquidy (about 5-8 mins).
Increase heat to medium-high and bring to a boil, stirring constantly.
Boil (still stirring constantly) for 1 minutes and remove from heat. Add remaining 2 cups of blueberries, butter, and vanilla extract and stir until butter is melted. Allow to cool while you prepare your crust.
To prepare your cookie crust, combine flour, sugars, baking powder, cornstarch, and salt in a food processor and pulse briefly until combined.
Sprinkle your butter pieces evenly over the dry ingredients and pulse until the mixture resembles coarse crumbs.
Add egg and vanilla extract and pulse again until mixture just begins to cling together.
Pour ½ of your cookie batter into the bottom of your prepared pan and use your hands or the back of a spoon to press evenly into the bottom of the pan to form a crust.
Pour your cooled blueberry filling evenly over the crust.
Use your hands to sprinkle the remaining crust mixture evenly over the top of the filling.
Transfer to the oven and bake on 350F (175C) for 55-60 minutes or until the top of the blueberry crumb bars is golden brown.
Allow to cool completely (2-3 hours) before slicing and serving. If you prefer to enjoy these warm the crumb bars WILL fall apart if sliced before they have cooled and set, so plan to use a spoon!
Store in an airtight container at room temperature for up to 2-3 days or in the refrigerator for up to five days.
This recipe has been successfully made using Cup4Cup gluten free flour. It was noted that the crust needed to bake longer than indicated in the recipe to become crisp.