Prepare the Crust and Topping:

Preheat your oven to 350°F (175°C) and line a 9x13-inch baking dish with parchment paper, leaving some overhang for easy removal.

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

Add the cold, cubed butter and use a pastry cutter or your fingers to work it into the dry ingredients until the mixture resembles coarse crumbs.

Add the egg and mix until combined. The dough will be crumbly but should hold together when pressed.

Press half of the crumb mixture evenly into the bottom of the prepared baking dish to form the crust. Reserve the other half for the topping.

Make the Lemon Filling:

In a medium bowl, whisk together the eggs, sugar, lemon juice, and lemon zest until smooth.

Gradually whisk in the flour until fully incorporated.

Assemble the Bars:

Pour the lemon filling evenly over the crust.

Sprinkle the reserved crumb mixture over the top of the lemon filling, ensuring an even layer.

Bake:

Bake in the preheated oven for 35–40 minutes, or until the top is golden brown and the filling is set.

Remove from the oven and let the bars cool completely in the pan.

Once cooled, lift the bars out of the pan using the parchment paper and transfer them to a cutting board. Slice into squares or rectangles.