In a heavy-duty stand mixer with a paddle attachment, cream the butter and sugar, about 30 seconds. Mix in the lemon zest, 1/2 teaspoon salt, and 1 cup flour until combined. Add the remaining flour and mix on low speed just until the dough begins to come together. Move the dough onto a lightly floured surface and pull the dough together into a ball with your hands. Cover with plastic wrap; refrigerate for 1 hour.
Preheat the oven to 250 degrees F.
Remove the dough from the refrigerator and unwrap it. Roll the dough into an 8 by 8-inch square, about 1/3-inch thick. Cut the dough into 16 (2-inch) squares and sprinkle them with salt.
Line a baking sheet with parchment paper and evenly space the little squares on the paper. Bake until light golden brown on the edges, 50 to 60 minutes. Allow to cool on the baking sheet. Serve at room temperature.