1.Preheat oven to 180°C/350°F. Grease 20cm x 30cm (8 inch x 12 inch) lamington pan; line base and sides with baking paper.
2.Beat butter and sugar in small bowl with electric mixer until light and fluffy. Add egg yolks, beat well.
3.Transfer mixture to large bowl; stir in sifted flour and soda in two batches, then nuts and yoghurt.
4.Beat egg whites in small bowl with electric mixer until soft peaks form. Gently fold egg whites into yoghurt mixture in two batches. Spread mixture into pan. Bake about 35 minutes.
5.Stand cake in pan 5 minutes before turning, top-side up, onto wire rack to cool; dust with sifted icing sugar before serving, if you like.
Store in an airtight container, at room temperature, for up to two days.Note