350²µ/10Β½΄Η³ϊ plain flour
Β½ tsp bicarbonate of soda
300²µ/10Β½΄Η³ϊ caster sugar
1-2 tbsp icing sugar, to taste
4 tsp vanilla extract
225g/8oz soft butter, plus extra for greasing
6 free-range eggs
250ml/9oz low-fat natural yoghurt