Cream the sugar, butter, and salt until light and fluffy. Stir in the vanilla extract, lemon juice, and lemon zest. Add the flour in thirds just until incorporated.
1 cup unsalted butter, softened, 1 cup confectioners' sugar, 1/2 teaspoon salt, 1/2 teaspoon vanilla extract, 1 tablespoon lemon zest, 1 tablespoon fresh lemon juice, 2 cups all-purpose flour