1.Preheat oven to 180°C (160°C fan-forced). Line 4 oven trays with baking paper.
2.Beat butter, vanilla and sugars in a small bowl with an electric mixer until pale and creamy. Beat in egg yolk.
3.Sift flour and baking powder together in a separate bowl.
4.Stir rinds into butter mixture then stir in flour mixture. Turn dough onto lightly floured surface; knead until it forms a smooth dough. Shape dough into a disc. Wrap dough in plastic; refrigerate for 2 hours.
5.Roll dough between sheets of baking paper until about 5mm thick. Cut out as many 7cm rounds from dough as possible (or the same size as your cookie stamp). Transfer rounds to trays; press with cookie stamps. Gather up dough scraps; knead lightly until smooth. Repeat.
6.Bake shortbread for 10-12 minutes or until browned lightly. Cool on trays. Dust with sifted icing sugar.
Suitable to freeze. Not suitable to microwave.
If you don’t have cookie stamps, you can cut the shortbread out with a fluted cutter. Shortbread will keep for up to 2 weeks in an airtight container or can be frozen for up to 2 months.