Preheat oven to 325°F (165°C). Line a baking tray with parchment paper.
Beat softened butter and powdered sugar until smooth and creamy.
Mix in vanilla extract, if using.
In a separate bowl, whisk together flour, cornstarch, and salt.
Gradually add dry ingredients to butter mixture and mix until a soft dough forms.
Shape dough into a disc, wrap in plastic wrap, and chill for 20–30 minutes.
Roll dough to about ½ inch (1.25cm) thick. Cut into rectangles, squares, or your preferred shapes.
Prick each biscuit with a fork.
Arrange on prepared tray. Bake for 18–22 minutes until edges are just lightly golden.
Let cool on tray for 5 minutes, then transfer to a wire rack to cool completely.
Dust with powdered sugar or decorate with melted chocolate if desired.
