Preheat your oven to 375 degrees F, and spray an 8x8 baking pan with cooking spray (one with flour in it works great) and set aside.
In a medium bowl, combine the blueberries, sugar, cornstarch and lemon juice for the blueberry filling, toss to coat, and set aside.
In the bowl of your stand mixer fitted with the paddle attachement (or large bowl if using a hand mixer), combine the flour, sugar, baking powder, butter and egg, and mix until the dough is moist but very crumbly. Place a little more than half of the dough into your prepared baking pan and press it down evenly, to create your crust.
Pour the blueberry filling mixture over the dough and then sprinkle the remaining, crumbly dough over the top. It doesn't need to cover every inch, it's ok if you can see blueberries through it! Bake for 35 – 45 minutes, or until the filling is bubbly and the dough has lightly browned. Let cool completely before cutting and eating!