Line a metal square 8x8 baking pan with parchment paper and preheat the oven to 350 F/180 C.
Cook the frozen blueberries, lemon zest, lemon juice, and sugar over medium heat for 8-9 minutes or until the berries start to release their juices and boil. Whisk together the cornstarch and water and stir it into the jam. Cook for 2 minutes longer until the jam is super thick. Transfer it to a bowl and set it aside to cool completely.
In a medium bowl, combine the flour, sugar, brown sugar, and melted butter until crumbs form. Chill in the fridge until ready to use. Before using, break up the crumb into small pieces!
In a large bowl, whisk together the brown sugar, granulated sugar, egg, egg yolk, and vanilla extract until light and fluffy, about 1 minute. Then whisk in the melted butter. Once combined, fold in the flour, baking powder, and salt.
Evenly spread the batter into the prepared pan.
Evenly spread the cooled blueberry jam over the top of the batter. Then sprinkle the crumb topping and extra blueberries over the top.
Bake for 28-30 minutes, or until the middle is puffy, the edges are golden brown, and a toothpick inserted into the center comes out covered with just a few moist crumbs.
Let the pan cool completely on a wire rack. Then lift the parchment paper up to remove it from the pan. Use a sharp knife to cut it into 16 small bars or 9 large bars. Enjoy!