Melt the chocolate in a bowl over a pan of simmering water (see
Place the remaining butter, sugar, eggs and vanilla extract in a bowl. Sift the flour and baking powder into the bowl and beat until light and fluffy.
Grease the base and long sides of a 1 kg (2 lb) loaf tin. Spoon one-quarter of the mixture into the tin. Spread one-third of the chocolate mixture over the cake mixture. Repeat the layering of the cake mixture and chocolate sauce, finishing with a layer of cake mixture. Sprinkle with the chopped chocolate.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 1¼ hours or until risen and a skewer inserted into the centre of the teabread comes out clean. Leave in the tin for 10 minutes, then transfer to a wire rack to cool.