Preheat oven to 350 degrees. Prepare an 8x8 baking pan using cooking spray or (very light oil). You could also use a layer of parchment paper, leaving sides overhanging for ease of removal later. I wish I would have done this.
Using a food processor, combine the ingredients for the shortbread layer. Pulse until you get a very fine crumb. It should be dry and similar to cornmeal.
Press the shortbread mixture into the bottom of the baking pan--being careful to have an even layer all around.
Bake for 15 minutes or until golden brown on top--which may take a bit longer depending on your oven.
Lemon Filling
While the shortbread layer is baking, prepare the lemon filling. Add eggs, sugar, lemon juice, lemon zest, and ¼ cup of flour to a stand mixer. Stir until well combined.
Pour the lemon filling, (it will be very runny--but don't worry) over the top of the shortbread layer.
Bake for an additional 25-30 minutes at 350 degrees.
Allow to cool to room temperature or colder (I like to refrigerate mine). Once cool, sprinkle with powdered sugar and enjoy!