Preparation Steps

• Preheat the oven to 325°F and position a rack in the lower-middle. Line a 9×13-inch metal or ceramic pan with a parchment sling, leaving overhang on the long sides.
• Make the crust: In a large bowl, beat the softened butter with powdered sugar until creamy, about 1 minute. Mix in almond extract, salt, and 1 teaspoon lemon zest. Add the flour and mix just until the dough looks sandy and holds together when squeezed.
• Press the dough firmly and evenly into the prepared pan (use the bottom of a measuring cup for a tight, level layer). Dock the surface all over with a fork, then freeze the pan for 10 minutes to help prevent shrinkage.
• Bake the crust for 24–26 minutes, until the edges are lightly golden and the surface looks set and matte.
• While the crust bakes, prepare the filling: In a large bowl, whisk the eggs and yolk gently just to break them up. Add the sugar and whisk until combined but not frothy. Whisk in lemon zest and salt, then the lemon juice.
• Sift the flour and cornstarch together directly over the bowl and whisk only until smooth. Let the filling sit for 5 minutes to allow bubbles to rise, then strain it into a pitcher to remove any lumps and excess foam.
• As soon as the crust comes out of the oven, give the filling a quick stir and pour it over the hot crust. Return the pan to the oven and bake at 325°F for 22–26 minutes, rotating once halfway, until the center wobbles slightly when nudged and the edges are set (do not overbake).
• Cool the pan on a rack for 1 hour, then refrigerate until fully chilled and firm, at least 2 hours (3 is ideal).
• Lift the slab out using the parchment. Dust with powdered sugar, trim the edges for clean sides, and cut into 24 bars. For the neatest cuts, warm a sharp knife under hot water and wipe it dry between slices.