1 x 397g tin caramel
1 tsp baking powder
175g/6oz caster sugar
250g/9oz icing sugar
55g/2oz light muscovado sugar
陆 tsp fine sea salt
225g/8oz self-raising flour
1 tsp vanilla extract
115g/4oz salted caramel fudge, chopped into small pieces
250g/9oz unsalted butter, very soft
4 free-range eggs
225g/8oz baking spread, from the fridge, plus extra for greasing
1 tbsp milk