CHOCOLATE AND SALTED CARAMEL TRAYBAKE

225g unsalted butter, softened, plus extra to grease • 200g Waitrose & Partners Light Brown Soft Sugar • 3 British Blacktail Free Range Eggs • 1 tsp vanilla extract • 70g dark chocolate, melted • 150ml buttermilk • 225g self-raising flour • ½ tsp bicarbonate of soda • 50g cocoa powder • ½ tsp fine salt TOPPING: 200g Cooks’ Ingredients Salted Caramel Icing • 30g dark chocolate, melted