Step 1Heat the oven to 350 degrees. Line the bottom and sides of a 13-by-9-inch cake pan with foil, leaving overhang on all sides, and lightly coat the foil with nonstick cooking spray.
Step 2Whisk the flour, ½ cup powdered sugar and ¾ teaspoon salt in a large bowl. Add the butter, toss to coat, and press into the dry ingredients using a pastry cutter or your fingers until fine crumbs form. Or, use a food processor and pulse the dry ingredients until mixed, then pulse in the butter until fine crumbs form. Scatter the crumbs evenly across the bottom of the prepared pan and press firmly into the bottom in an even layer. Bake until the bottom crust is golden brown, 15 to 17 minutes.
Step 3While the crust bakes, make the filling: Finely grate 1 tablespoon zest from the lemons into a large bowl, then squeeze ½ cup juice into the bowl. (You may not need all 4 lemons.) Add the eggs, labneh, remaining ¾ cup powdered sugar and remaining ¼ teaspoon salt and whisk slowly until smooth. You don’t want to create too many tiny bubbles while whisking or the top will look cratered after baking.
Step 4Remove the pan from the oven, pour the lemon filling over the hot crust and return the pan to the oven. Reduce the oven temperature to 300 degrees. Bake until set around edges and barely jiggly in the center, 15 to 20 minutes.
Step 5Cool the bars completely on a wire rack, then refrigerate until firm, at least 2 hours. Using the overhanging foil, lift and slide the bars onto a cutting board and cut into 24 bars.
Berry Lemon Bars
Arrange raspberries, blackberries, blueberries or cut strawberries over the chilled bars.
Stone Fruit Lemon Bars
Arrange peeled and sliced peaches, nectarines or apricots over the chilled bars.