Preheat the oven to 170 degrees C. In a medium bowl mix together the flour, 1/3 cup of icing sugar and butter until butter is in very small crumbs. Press into the bottom of a 20x30cm pan.
Bake for 25 minutes in the preheated oven or until firm and lightly browned. While the crust is baking whisk together the eggs, 2/3 cup of icing sugar, lemon juice and lemon zest in a medium bowl. Just before the crust is done, stir in the baking powder. Pour over the crust while it is hot and return to the oven.
Bake for another 25 minutes in the preheated oven until set. Dust with remaining icing sugar when cool.