Preheat the oven to 160°C (320°F) and line a baking sheet with parchment paper.
Cream the room-temperature butter and granulated sugar together until pale and light using a mixer or wooden spoon.
Add vanilla extract if using and mix just to combine.
Sift the all-purpose flour, cornstarch (if using) and sea salt into the butter mixture.
Fold the dry ingredients into the butter with a spatula until a soft dough forms; avoid overmixing.
Turn the dough onto a lightly floured surface and gently shape into a disc; chill for 30–60 minutes to firm.
Roll out the dough to about 1/2 inch (12 mm) thickness or press into a band and cut rounds with a cookie cutter.
Place cookies on prepared sheet leaving space between cookies for slight spread.
Optional: prick each cookie lightly with a fork for a classic look.
Bake for 12–15 minutes until edges are very lightly golden; centers should remain pale.
Cool on the sheet 5 minutes, then transfer to a wire rack to cool completely.
Store cooled cookies in an airtight container; they firm up and develop flavor after a few hours.