Preheat your own to 350℉/176℃/Gas Mark 4.
Grease a 9”x13” pan.
In a large bowl, whisk together the all-purpose flour, 3/4 c. granulated sugar, baking powder, and salt.
Cube the cold, unsalted butter and cut it into the flour mixture using either a pastry cutter or food processor. Add the egg and vanilla extract and mix in.
Press half of the mixture into the base of the pan.
In a medium bowl, pour your fresh, washed blueberries into it. Add the 1/4 c. granulated sugar and cornstarch and mix until the blueberries are fully coated.
Pour the blueberries over the mixture in the pan, making sure that they are laid out evenly.
Lastly, spread the rest of the crumble topping on top of the blueberries.
Bake for 1 hour or until the edges of the bars are lovely golden-brown colour.
Remove from the oven and let cool fully before slicing the bars and serving.