Traditional Scottish Shortbread (Shortbread Écossais Traditionnel)

1.Preheat your oven to 325°F (160°C). Lightly grease an 8x8 inch square baking pan or line it with parchment paper, allowing the paper to overhang slightly for easy removal.

2.In a large bowl, cream together the softened butter, confectioners' sugar, and granulated sugar using an electric mixer until the mixture is pale, light, and fluffy (about 3-4 minutes). If using, mix in the vanilla extract.

3.In a separate medium bowl, whisk together the sifted flour, cornstarch, and salt until thoroughly combined.

4.Gradually add the dry ingredients to the wet ingredients in three additions, mixing on low speed until just combined. Be careful not to overmix; the dough will be soft and slightly crumbly.

5.Press the dough evenly into the prepared baking pan, ensuring the surface is smooth and compact. Use the bottom of a measuring cup or glass to help flatten it firmly.

6.Using a sharp knife, lightly score the dough into 16 squares (4x4 pattern) without cutting all the way through. Prick the surface of the dough several times with a fork to prevent bubbling.

7.Bake for 35 to 45 minutes, or until the edges are set and very lightly golden brown. The center should still look pale.

8.Immediately upon removing the shortbread from the oven, sprinkle the top lightly with extra granulated sugar, if desired. While the shortbread is still hot, re-cut along the scored lines.

9.Allow the shortbread to cool completely in the pan on a wire rack (this is crucial for texture). Once fully cooled, lift the shortbread out using the parchment paper overhang and break or cut along the score lines.

10.Store in an airtight container at room temperature for up to one week.