Lemon and Cardamom Loaf

50g butter

275g golden caster sugar

3 eggs, separated

225g natural yoghurt

1 tbsp lemon juice and the zest of 3 lemons

175g self-raising flour

1 tsp baking powder, sieved

1/2 tsp ground cardamom

To ice:

125g icing sugar

1 ½ tbsp lemon juice

a handful pistachio nuts, finely chopped