Combine the flour, sugar, baking powder, and salt in a medium bowl.
Mix the cold butter into the dry ingredients using a pastry cutter, a fork, or your fingers until only small chunks of butter remain. Add egg yolk and cold water and stir until the mixture is wet and loosely sticking together. It will be crumbly.
Take half of the cobble mixture and press into the prepared pan to form the crust. The rest should be saved for the top of the bars.
Prepare the filling by adding the blueberries, lemon juice, flour, sugar, cinnamon and vanilla to a bowl. Stir until well combined.
Spoon the blueberry filling evenly over the crust. Then sprinkle the remaining crumble over the top of the blueberry filling.
Bake for 30-35 minutes or until golden brown and set.