Preheat your oven to 350°F (175°C). Line an 8x8 inch baking pan with parchment paper, leaving an overhang on the sides to lift out the bars easily after baking.

In a large bowl, mix together 1 1/2 cups flour, 1/2 cup sugar, baking powder, and salt. Add the cubed cold butter and use a pastry cutter or your fingers to blend it into the dry ingredients until the mixture resembles coarse crumbs.

Stir in the egg to form a crumbly dough.

Press about two-thirds of this mixture into the bottom of the prepared pan to form a firm, even layer. Reserve the remaining mixture for the topping.

Bake the crust in the preheated oven for 15-20 minutes, or until lightly golden. Remove from the oven and let cool slightly while you prepare the filling.

Zest and juice the lemons to get approximately 2 teaspoons of zest and 1/2 cup of juice.

In a bowl, whisk together 1 cup sugar, 2 tablespoons flour, lemon zest, lemon juice, and 2 eggs until well combined.

Pour the lemon filling over the pre-baked crust, spreading it evenly.

Crumble the remaining dough mixture over the lemon filling to create a crumb topping.

Return to the oven and bake for an additional 20-25 minutes, or until the filling is set and the topping is lightly golden.

Let the bars cool in the pan on a wire rack. Chill in the refrigerator for at least 2 hours to set before cutting into squares.

Lift the bars out of the pan using the parchment paper overhang and cut them into squares.