1.Preheat oven to 170°C (150°C fan-forced). Grease a deep 27cm-round cake pan and line base with baking paper.
2.Beat butter, rind and sugar in a large bowl with electric mixer until light and fluffy. Beat in eggs one at a time. Stir in half the sifted flours and half the sour cream, then stir in remaining flours and remaining sour cream until smooth.
3.Spread mixture into pan and bake about 1½ hours. Stand cake 5 minutes before turning onto wire rack to cool. Dust with sifted icing sugar before serving, if desired.