1.Preheat oven to 180°C (160°C fan-forced). Grease 20cm baba pan thoroughly (or grease deep 20cm-round cake pan and line base and side with baking paper).
2.Beat butter, rind and sugar in medium bowl with electric mixer until light and fluffy. Beat in eggs, one at a time. Stir in sifted flour and remaining ingredients, in two batches.
3.Spread mixture into pan; bake about 45 minutes. Stand cake in pan 5 minutes; turn onto wire rack over tray.
4.Meanwhile, make lime syrup. Stir ingredients in small saucepan over low heat, without boiling, until sugar dissolves. Simmer, uncovered, without stirring, 3 minutes.
5.Pour hot syrup over hot cake.