Heat the oven to 350°F. Grease a 9-inch square pan. Combine 2 cups of the flour, the powdered sugar, and the salt in a large bowl. Add the butter and blend with a pastry cutter or your fingers until the mixture resembles coarse meal. (You can also do this in a food processor, but don’t overprocess it.) Press into the greased pan, pushing the dough all the way up the sides. Bake until the edges are golden brown, about 20 minutes, then remove and reduce the oven temperature to 315°F.
Meanwhile, in another large bowl, whisk together the eggs and sugar until smooth. Gently stir in the lemon juice and zest. (To minimize aesthetically displeasing little bubbles on the top of the bars, avoid whisking further.) Fold in the remaining ½ cup flour.
Pour the egg mixture over the hot crust and bake until the curd is set and no longer jiggles when you move the pan, 35 to 45 minutes. Cool thoroughly before cutting into bars. Dust with powdered sugar and serve.
