Preheat your oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment paper, letting the paper hang over two sides for easy removal. This should take 2–3 minutes.

In a medium bowl, combine 2 cups all-purpose flour, 1/2 cup granulated sugar, and 1/2 teaspoon salt. Stir with a fork until evenly mixed, about 1 minute. The mixture will look like fine sand.

Cut 1 cup cold unsalted butter into small cubes (approximately 1/2-inch pieces). Add the cubes to the flour mixture immediately. Using a pastry cutter, two knives, or your fingertips, work the butter into the flour until the mixture resembles coarse breadcrumbs with pea-sized pieces of butter visible. This should take 3–5 minutes. Do not overmix or the crust will be tough.

Press the shortbread mixture firmly and evenly into the bottom of your prepared pan using your fingers or the bottom of a measuring cup. Make sure the layer is compact and reaches into all corners. The crust should be about 1/4-inch thick and feel solid when pressed.

Bake the crust alone for 12–15 minutes at 350°F until it turns light golden brown around the edges and feels set to the touch. You should smell a buttery aroma. Checkpoint: The crust should be lightly colored but not dark; remove it when the edges are barely golden.

While the crust bakes, prepare the filling. In a medium mixing bowl, crack 4 large eggs and separate the yolks from the whites, discarding the whites (or save for another use). Add the 4 egg yolks to a clean bowl.

Pour 1 14-ounce can of sweetened condensed milk into the bowl with the egg yolks. Add 1/2 cup fresh key lime juice, 2 tablespoons fresh regular lime juice, 1 tablespoon key lime zest, and 1 teaspoon vanilla extract.

Using a whisk or electric mixer on medium speed, blend the filling ingredients together for 2–3 minutes until smooth, pale, and well combined. The mixture should look like a thick, creamy custard with no lumps. Scrape down the sides of the bowl with a spatula to ensure even mixing.

Remove the partially baked crust from the oven (it should still be warm). Carefully pour the key lime filling over the warm crust in an even layer. Use a spatula to spread it gently if needed, but do not overmix.

Return the pan to the oven and bake for 15–18 minutes at 350°F. The filling should be mostly set but still have a very slight jiggle in the center when you gently shake the pan—similar to the texture of slightly underbaked cheesecake. The edges should look set and firm. Checkpoint: The filling is ready when the edges are firm but the very center still jiggles slightly (about 1-inch diameter in the middle is acceptable).

Remove the pan from the oven and allow it to cool to room temperature on a wire rack, about 45–60 minutes. The bars will continue to set as they cool.

Once cooled to room temperature, cover the pan loosely with plastic wrap and refrigerate for at least 4 hours, preferably overnight. This resting time allows the flavors to meld and the bars to firm up completely, making them easier to cut cleanly.

To serve, lift the parchment paper on both sides to remove the entire bar slab from the pan and transfer to a cutting board. Using a sharp knife dipped in hot water and wiped clean between cuts, slice into 2-inch squares (you should get 18–24 bars depending on size).

Dust each bar lightly with powdered sugar using a fine-mesh sieve. Serve chilled or at room temperature with a dollop of whipped cream and an optional thin lime wheel for garnish.