Preheat oven to 350 degrees. Line an 8 X 8" square pan with parchment paper.
Pulse flour, sugar and butter in food processor until soft dough forms. Press into bottom of lined pan with a fork.
Bake crust 20 minutes.
While crust is baking add zest, juice, sugar, flour, salt and eggs to food processor. Pulse until well mixed and slightly foamy.
When crust has finished first 20 minutes, pulse filling ingredients one last time and pour into the crust. Return to the oven for 25-30 minutes. Make sure filling in the center of the pan is set.
Remove from oven and allow to cool completely. Dust with remaining icing sugar. Run a sharp knife around outer edge of the pan to loosen the sides. Cut into uniform squares and transfer to a serving plate.
Store in refrigerator well covered.