1. Preheat oven to 350°F (175°C). Line a 9×13-inch baking pan with parchment paper. In a medium bowl, whisk together flour, baking powder, and salt. Set aside.

2. In a large bowl, cream softened butter and ¾ cup granulated sugar on medium-high speed for 2-3 minutes until light and fluffy.

3. Reduce mixer speed to low, add egg yolk, 1 tsp vanilla extract, and 1 tbsp lemon zest. Mix until incorporated, scraping bowl sides.

4. With mixer on low, gradually add dry ingredient mixture to wet, mixing until just combined and no dry streaks remain. Dough will be thick and crumbly.

5. Press two-thirds of the dough evenly into the prepared pan. Bake for 15-18 minutes, until edges are lightly golden brown. Set aside remaining dough.

6. In a medium saucepan, combine blueberries, ½ cup granulated sugar, 2 tbsp fresh lemon juice, and 1 tbsp lemon zest. Stir to combine.

7. Cook over medium heat, stirring occasionally, for 5-7 minutes until blueberries soften and release juices. Gently mash some berries.

8. In a small bowl, whisk 3 tbsp cornstarch and 3 tbsp cold water to form a smooth slurry. Pour into simmering blueberry mixture, stirring constantly. Cook for 1-2 minutes until filling thickens to a jam-like consistency. Remove from heat.

9. Stir in ½ tsp pure vanilla extract into the filling. Allow the blueberry lemon filling to cool slightly.

10. Spread the slightly cooled blueberry lemon filling evenly over the warm pre-baked crust. Crumble the remaining one-third of the cookie dough evenly over the filling.

11. Return the pan to the preheated 350°F (175°C) oven. Bake for another 30-35 minutes, until the crumb topping is golden brown and the filling is bubbly.

12. Remove from oven and place on a wire rack to cool completely for 2-3 hours at room temperature. Cooling completely is essential for the filling to set properly, making the bars easy to cut.

13. If desired, whisk 1 ½ cups sifted powdered sugar and 2-3 tbsp fresh lemon juice in a small bowl until a pourable but thick consistency is reached. Adjust with more lemon juice or powdered sugar as needed.

14. Once bars are completely cool, lift them from the pan using the parchment paper overhangs. Drizzle the lemon glaze evenly over the top and let it set for 15-20 minutes. Use a sharp knife to cut into 12-16 squares/rectangles and serve.