1.Preheat oven to 150°C. Grease two oven trays. Beat butter and sugar in a small bowl with an electric mixer until combined.
2.Sift flour. Add vanilla extract, lime rind and fresh basil leaves with the flour. Add large spoonfuls of flour mixture to butter mixture, beating after each addition. Press ingredients together gently; knead on a lightly floured surface until smooth.
3.Divide dough into two portions. Shape portions into 18cm (71/4in) rounds.
4.Place rounds on trays; mark into wedges, then prick with a fork. Pinch a decorative edge with floured fingers.
5.Bake shortbread for 30 minutes. Leave shortbread on trays for 10 minutes before transferring to wire racks to cool.
6.Sprinkle the baked shortbread rounds with 1 tablespoon each extra caster sugar while still warm.
You will need 2 limes for the recipe.
Cut when cold. Recipe can be made a month ahead; store in an airtight container.Note