Preheat the oven to 350°F (175°C), and line a 9x13-inch (33x23 cm) baking dish with parchment paper.
In a large bowl, cream the butter and sugar together until light and fluffy, about 2–3 minutes.
1 cup unsalted butter, 1 ½ cups granulated sugar
Add the egg, lemon juice, lemon zest, and vanilla extract. Mix until fully combined. The mixture may look slightly curdled from the lemon juice. This is normal.
1 large egg, 1 tablespoon lemon zest, 2 tablespoons fresh lemon juice, ½ teaspoon vanilla extract
Add the flour, salt, and baking soda. Mix just until the dough comes together. Don’t overmix.
2 ½ cups all-purpose flour, 1 teaspoon salt, ½ teaspoon baking soda
Press the dough evenly into the prepared pan. It will be thick, so use a spatula or your hands to spread it into the corners.
Bake for 20–24 minutes, or until the edges are lightly browned and the center looks set. Let the bars cool completely in the pan.