Preheat the oven to 350℉. Line your square pan with parchment paper.
In a stand mixer, beat the butter on medium speed until whipped, about 1 minute. Then add the sugar, flour, salt and vanilla. Beat on medium speed until the dough comes together. It will be crumbly but pliable (about 1-2 minutes).
¼ cup granulated sugar, 2 tsp vanilla extract, 1 cup all purpose flour, ½ tsp salt, ½ cup unsalted butter
Dump the shortbread into the pan and press it into the edges with your fingers or a spatula until it's even. Then bake the pan in the oven for 20 minutes. Once finished, remove the shortbread from the oven and reduce the heat to 325℉.
While the shortbread is baking, make the lemon layer. In a large bowl, whisk together the eggs, lemon juice and lemon zest until smooth. Then add the sugar and flour. Whisk for a while until no lumps remain.
1 cup granulated sugar, 3 eggs, 3 large lemons, 1 tbsp all purpose flour
While the shortbread is still warm, pour the lemon filling into the pan. Tilt the pan until the layer covers the shortbread evenly.
Place it back into the oven to bake for about 20 minutes, or until the center of the bars look just set (barely wiggling). Then remove the bars from the oven and let cool for 10-20 minutes. Then place them in the fridge to chill for about 4 hours or longer.
Dust the bars with powdered sugar and slice into squares. Enjoy!
powdered sugar