Preheat oven to 375°F (190°C). Line a 9x13 inch baking pan with parchment paper, leaving an overhang on the long sides to easily lift the bars later.

In a large mixing bowl, whisk together the flour, granulated sugar, baking powder, and salt.

Add the cold, cubed butter to the flour mixture. Using your fingertips or a pastry blender, cut the butter into the dry ingredients until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining.

Stir in the egg yolk until just combined and the mixture forms larger, moist crumbs. Do not overmix.

Press about two-thirds (approximately 2 cups) of the crumb mixture evenly into the bottom of the prepared baking pan. Reserve the remaining crumb mixture for the topping. Bake the crust for 15 minutes, or until lightly golden.

While the crust bakes, in a medium saucepan, whisk together the granulated sugar, flour, cornstarch, and salt for the filling.

Gradually whisk in the water and lemon juice until smooth.

Cook over medium heat, stirring constantly with a whisk, until the mixture thickens and comes to a boil. Reduce heat to low and continue to cook, stirring, for 1 minute.

Remove from heat. In a small bowl, lightly beat the egg yolks. Gradually whisk about half a cup of the hot lemon mixture into the beaten egg yolks to temper them, then slowly pour the tempered egg yolks back into the saucepan, whisking constantly.

Return the saucepan to low heat and cook, stirring constantly, for another 1-2 minutes, until the filling thickens slightly more. Do not boil after adding the egg yolks.

Remove from heat and stir in the lemon zest and 2 tablespoons of unsalted butter until the butter is melted and incorporated.

Pour the hot lemon filling evenly over the partially baked crust.

Crumble the reserved crumb mixture over the lemon filling.

Return the pan to the oven and bake for another 20-25 minutes, or until the crumb topping is golden brown and the filling is set (it may still be slightly jiggly in the center, but will firm up as it cools).

Let the bars cool completely in the pan on a wire rack before lifting them out using the parchment paper overhang. This is crucial for clean cuts.

Once completely cooled, cut into 16 squares or bars.