Grease a 1 kg (2 lb) loaf tin lightly with butter and line the base with nonstick baking paper. Beat the butter, sugar and vanilla extract together in a bowl until smooth and creamy. Add the eggs and sift over the flour and baking powder. Beat together until smooth and creamy.
Add the bananas, milk and chopped chocolate and fold together until well mixed. Transfer the mixture to the prepared tin and bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 55–60 minutes until the cake is well risen and golden.
Cool the cake in the tin for 10 minutes before turning out on to a wire rack to cool completely. Serve cut into slices.