Preheat your oven to 350°F (175°C) and lightly grease with shortening or butter the sides and bottom of a 9×13 inch glass baking dish. Set aside.

For the Blueberry Jam: In a medium saucepan, combine the blueberries, granulated sugar, cornstarch, and lemon juice, and cook over medium heat, stirring occasionally, until the mixture is bubbling and the fruit has broken down 5 to 6 minutes. In a small bowl, mix about 2 tablespoons of the fruit mixture with the cornstarch until smooth. Add the cornstarch mixture to the fruit and simmer for another 2 to 3 minutes. Add the butter and stir until melted. Remove the blueberry jam from the heat and let it cool slightly.

For the Crumble Mixture for the Crust and Topping: In a food processor fitted with a steel blade, pulse flour, granulated sugar, light brown sugar, baking powder, and kosher salt to combine; add the chilled butter pieces and pulse until the mixture resembles a coarse crumble with only a few larger pieces, about 8 to 12 pulses. In a small bowl, beat the egg, egg yolk, water, and vanilla extract with a fork. (Do not beat until fluffy, you don’t need to incorporate air into the dough.)

Add the egg mixture down the feed tube and pulse quickly until the mixture is evenly moistened and crumbly; don’t let the dough form into a ball in the machine; the dough should hold together if you press it together with your fingers.

Scatter two-thirds of the dough in clumps in a 9 x 13-inch baking pan. Cover the dough with a piece of plastic wrap. Take a 9” x 5” loaf pan and press it gently onto the plastic-wrapped dough. Apply even pressure to flatten the dough, ensuring it spreads out and takes the shape of the pan.

Continue pressing the loaf pan onto the dough while gently moving it in different directions, flattening the dough evenly and extending it ½ inch up the sides of the pan. Repeat this process until all the dough has been flattened and extends ½ inch up the sides of the pan. Once flattened, refrigerate the dough in the pan and the remaining dough for 20 minutes. If you’re in a rush, you can proceed to the next step of your recipe without refrigerating the dough.

Spread the blueberry jam evenly over the crust, leaving a ½-inch border. Next, pinch medium pieces of the remaining dough with your fingers and drop them evenly on top of the blueberry jam; the dough will not cover the entire surface. Bake for 45 minutes or until the topping is golden brown. Allow to cool for 1-2 hours before slicing and serving.