Have ready a 9-inch (20x20cm) square baking pan that has been buttered or sprayed with baking spray.
Line it with parchment paper: take a large piece of parchment paper of about 8 inches (almost as wide as the pan) and cover the bottom and two sides of the pan. The other two sides will remain buttered but not lined with paper. This will help you remove the shortbread after it's baked.
In a large bowl, mix ¾ cup unsalted butter with 2 tablespoons lemon zest (I zest it directly over the butter mixture using a microplane zester) and ⅛ teaspoon vanilla extract. Add ½ cup powdered sugar and incorporate well so there are no lumps. I recommend sifting the sugar before adding it, or sifting it directly over the butter mixture as I do.
Add 1 ¼ cups all-purpose flour and pinch of salt in 2 parts, mixing with the spatula until it is smooth, well combined and no dry spots remain.
Transfer the dough to the prepared pan, scattering small amounts on the bottom.
Lightly pat the dough with your hand until it covers the whole surface. Make sure the edges are covered. Make it as even as possible, but relax because it will not be completely smooth.
Refrigerate the whole pan, covered in plastic wrap, for at least 1 hour (and up to 24 hours). You can bake it directly, but this resting time creates a more tender shortbread, in my experience.
Preheat the oven to 325°F (160°C).
Sprinkle 1 tablespoon granulated sugar over the top before baking.
Bake the shortbread for 40-45 minutes, until the top is dry, has puffed slightly and is beginning to color. You can leave it a few more minutes for a more golden shortbread. Just don’t let it dry out.
Let cool for 10 minutes on a wire rack and carefully cut into sticks, squares or triangles with a smooth-bladed knife.
Let cool on the cooling rack completely, lift the whole square of the pan with the aid of the parchment paper, and carefully break each piece.
Keep in airtight cookie jars or tins.