Lemony Christmas Shortbread Cookies

2/3 cup (150g) unsalted butter, softened
1/3 cup + 1 tablespoon (55g) icing sugar, sifted
¼ cup (60ml) lemon juice
finely grated zest of 2 lemons
1 teaspoon vanilla extract
1 ¾ cups + 1 tablespoon (255g) all purpose flour, sifted
2 tablespoons corn starch, sifted
2/3 cup (94g) icing sugar, extra, sifted

Place the butter and icing sugar in the bowl of an electric mixer and beat for 8 minutes or until pale and creamy. Add the lemon juice, zest and vanilla and beat until combined. Add the flour and corn starch and beat just until a smooth dough forms. Turn dough onto a lightly floured surface and divide in half. Place each half onto a large piece of baking paper form into a 20cm (8in) long log. Wrap well in the paper and refrigerate for 1 ½ hours or until firm.
Preheat the oven to 180°C/350°F. Line two large baking sheets with baking paper.
Unwrap one of the dough logs and slice it into 1cm-thick rounds. Place onto the prepared baking sheets 5cm (2in) apart and bake for 15-18 minutes or until light golden. Remove from the oven and cool in the sheets for 5 minutes.
 Sprinkle the warm shortbreads with the extra icing sugar and allow to cool completely on wire racks. Should make about two dozen.