350²µ/10½´Ç³ú plain flour
½ tsp bicarbonate of soda
300²µ/10½´Ç³ú 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