Preheat your oven to 350°F (180°C). Prepare an 8-inch square baking dish by greasing it with butter or baking spray. For easier removal, line the base and two opposite sides with parchment paper, leaving an overhang for lifting.
In a mixing bowl, cream the softened butter with the granulated sugar until the mixture is smooth and uniform.
Beat in the eggs, one at a time, stirring just until incorporated. Avoid overmixing to maintain a soft texture.
Stir in the lemon zest and vanilla extract, blending gently to combine.
Sift together the flour and salt, then gradually fold them into the butter mixture. Mix only until no dry streaks remain.
Incorporate the lemon juice, stirring briefly until just combined. Do not overmix.
Transfer the batter into the prepared pan, smoothing out the surface evenly with a spatula.
Place in the preheated oven and bake for approximately 20 minutes, or until the center is just set. A toothpick inserted should come out with a few moist crumbs.
Remove from the oven and allow the pan to cool on a wire rack for 10 to 15 minutes. Drape a clean kitchen towel loosely over the top while cooling to retain moisture. Let cool almost entirely before glazing.
Once cooled, run a sharp knife along the unlined edges to loosen the blondies, then lift them from the pan using the parchment paper. Place on a cutting board and slice into squares.