Step 1: Preheat your oven to 350°F (175°C). Line an 8×8 or 9×9 inch baking pan with parchment paper, leaving an overhang on the sides.
Step 2: In a medium bowl, whisk together the all-purpose flour, granulated sugar, and salt for the crust.
Step 3: Add the cold, cubed unsalted butter to the dry ingredients. Using your fingertips, a pastry blender, or a food processor, cut the butter into the flour mixture until it resembles coarse crumbs.
Step 4: Press the mixture evenly into the bottom of the prepared baking pan. Use the bottom of a flat glass or a measuring cup to get a compact, even layer.
Step 5: Bake the crust for 18-20 minutes, or until the edges are lightly golden brown. While the crust is baking, prepare the lemon filling.
Step 6: In a separate large bowl, whisk together the the granulated sugar and all-purpose flour for the filling until well combined.
Step 7: Add the large eggs, fresh lemon juice, and lemon zest to the sugar-flour mixture. Whisk vigorously until smooth. Then, pour in the melted butter and whisk until fully incorporated.
Step 8: Carefully pour the lemon filling over the hot, pre-baked shortbread crust. Return the pan to the oven.
Step 9: Bake for an additional 20-25 minutes, or until the center is just set but still slightly wobbly when gently shaken. The edges should be lightly golden.
Step 10: Remove the pan from the oven and let the lemon bars cool completely on a wire rack at room temperature. Once cooled, refrigerate for at least 2-3 hours (or overnight) before slicing.
Step 11: Once thoroughly chilled, use the parchment paper overhang to lift the entire slab of lemon bars out of the pan. Place on a cutting board and slice into squares or rectangles using a sharp, warm knife.
Step 12: Dust generously with powdered sugar just before serving. Enjoy!