1.Grease a 23cm square slab pan, line base with paper; grease paper.
2.Beat butter, rind and sugar in small bowl with electric mixer until light and fluffy; beat in eggs one at a time, beat mixture until combined, stir in yoghurt, apple and currants, then sifted dry ingredients.
3.Spread mixture into prepared pan, bake in moderate oven for about 55 minutes. Stand 5 minutes before turning on to wire rack to cool. Dust with sifted icing sugar before serving.
Keeping time: 4 days.Note