For the Shortbread Crust: Preheat the oven to 350°F. Line an 8x8 square baking pan with parchment paper. Be sure to leave an overhang on the sides for easy removal.
In a food processor (preferred) or a medium bowl, combine the flour, sugar and salt. Add the cold, cubed butter. Pulse or use a pastry cutter until the mixture resembles coarse crumbs. Using a food processor is best here as it keeps the butter cold, which is the secret to the best shortbread crust texture!
Mix the egg yolk and water together, then drizzle over the flour mixture. Mix just until incorporated. It will look dry, but will hold its shape when you squeeze a small hanful in your palm. It it's too dry to hold together, add 1 more tablespoon of water.
Press firmly into the lined pan. It won't look like a smooth dough yet, but it will come together once you press it firmly and evenly into the bottom of the pan. Bake for 15-20 minutes, or just until the edges are lightly browned.
For the Lemon Filling: In a medium bowl whisk together the sugar, eggs, and lemon zest. Whisk in the lemon juice and flour until the mixture is completely smooth.
Pour over the baked crust and return to the oven to bake for 20-25 minutes. The bars are done when the edges are set and the center has only a very slight jiggle when the pan is tapped.
Cool completely before lifting them from the pan. Dust with powdered sugar then cut into squares. Enjoy!