THE BEST CHOCOLATE CAKES

125ml sunflower oil
125g butter
125ml cocoa
250ml water
400g light brown sugar
300g cake flour
1 tsp bicarb pinch of salt
125ml plain yoghurt
2 eggs
5ml vanilla extract

FOR THE ICING

200g icing sugar
2 Tbsp cocoa
75g butter, softened
3 Tbsp yoghurt
5ml vanilla extract
100g dark chocolate (I use Lindt 70%, but Bournville would be fine)