Preheat the oven to 350°F (175°C).

Grease the 9x9 inch baking pan.

In a mixing bowl, combine the flour, powdered sugar, and softened butter until resembling coarse crumbs.

Press the crumb mixture evenly into the bottom of the prepared pan.

Bake the crust for about 15 minutes, or until lightly golden.

In a separate bowl, whisk together the granulated sugar, eggs, lemon juice, lemon zest, baking powder, and salt.

Once the crust is slightly cooled, pour the lemon filling over it.

Bake again for another 20-25 minutes, until the filling is set.

Let the squares cool completely before slicing.