Preheat the oven to 325F/165C and line 2 cookie sheets with parchment paper.
Beat the butter and sugar until light and fluffy, about 3 minutes.
Beat in the vanilla extract.
In a separate bowl, sift the flour, cornstarch, and salt and add it to the butter, mixing until blended and soft.
Spoon the dough into a piping bag fitted with a 2D piping tip. Pipe the cookies about 1 1/4 inches across onto the prepared cookie sheets leaving about 2 inches between them.
Bake for 20 minutes or until the cookies just begin to brown lightly on the edges. Cool the cookies on the trays before storing in an airtight container.
The cookies will keep up to a week.
Shortbread dough is a thick, drier dough. If the dough doesn't come together, or the dough is too thick to pipe (it shouldn't be super easy to pipe) add a tablespoon or two of milk to loosen.For a Thicker Cookie that will hold it's shape more than mine, chill the unbaked cookies for 20 minutes before baking.The key to this recipe is good quality butter. If you can find it, try to find European Style butter. It has a higher milk fat content than ordinary butter.The essence of these cookies is the crisp, buttery indulgence that comes from the sweet cream flavor you get with good quality butter. Make sure the butter is softened before starting. Softened butter will ensure the butter creams well with the sugar.
Alternatives to Piping
If you don't have the piping bag, you can also use a cookie press instead.This dough can also easily be refrigerated and rolled to make cut out cookies. Just refrigerate the dough for 30 minutes and roll it out.