carrot cake

150g unsalted butter, softened, plus extra for greasing 300ml sunflower oil 100g dark brown soft sugar 250g light muscovado sugar 3 large free-range eggs 175g plain flour, sifted ¾ tsp baking powder ¾ tsp bicarbonate of soda 1 tsp ground cinnamon 225g carrots, grated 75g walnut halves, chopped...