1.Have butter at room temperature. Combine rum and raisins in small bowl; cover, stand about 1 hour or until rum is absorbed.
2.Preheat oven to 150°C.
3.Beat butter and sugar in small bowl with electric mixer until light and fluffy. Transfer mixture to large bowl; stir in half the sifted flours, raisin mixture, then remaining sifted flours. Knead dough on floured surface until smooth. Divide dough in half.
4.Roll one portion of dough between sheets of baking paper into a 23cm round. Pinch edges with floured fingertips. Mark round into wedges; prick wedges with a fork. Sprinkle with extra sugar. Slide round, still on baking paper, onto tray. Repeat with remaining dough.
5.Bake shortbread about 20 minutes. Cool on trays before cutting into wedges.
Shortbread will keep in an airtight container at room temperature for up to 2 weeks.