Mix together the flour, cocoa powder, bicarbonate of soda, baking powder and sugar in a bowl. Beat together the egg, milk and vanilla in a small bowl.
Put the butter, chocolate and measurement water in a saucepan and heat very gently, stirring frequently, until melted and smooth. Add the egg mixture and melted butter and chocolate mixture to the dry ingredients and stir well to mix.
Spoon the mixture into a greased and lined 23 cm (9 inch) round or 20 cm (8 inch) square cake tin (see
Make the topping. Beat together the butter and caramel or toffee sauce (see below) in a bowl. Transfer the cake to a serving plate, spread the topping over the top of the cake and scatter with chocolate curls.