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1
Heat the oven to 350 degrees F and arrange a rack in the middle.
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2
Coat a 13-by-9-inch baking dish with butter; set aside.
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3
Place 2 cups of the flour, the powdered sugar, and 1/4 teaspoon of the salt in the bowl of a food processor fitted with a blade attachment and pulse briefly to combine.
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4
Scatter the butter pieces over the flour mixture and pulse until the dough just begins to come together, about 20 (1-second) pulses.
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5
Crumble the dough into the prepared baking dish and, using the base of a measuring cup or a glass dipped in flour, press evenly into the pan.
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6
Bake until the crust is fully set and just starting to brown on the edges, about 25 minutes.
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7
Remove from the oven and let cool while you make the filling, at least 5 minutes.
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8
Reduce the oven temperature to 325 degrees F.
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9
To make the filling, combine the remaining 3 tablespoons flour, the remaining 1/4 teaspoon salt, the granulated sugar, and the eggs in a large bowl and whisk until the sugar has dissolved and the mixture is smooth, about 2 minutes.
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10
Add the lemon juice and zest and whisk until well combined.
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11
Pour the filling over the baked crust, return to the oven, and bake until set, about 25 to 35 minutes.
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12
Remove from the oven and let cool on a wire rack.
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13
Cut into squares and serve.
