Preheat the oven to 180C (350F). Line a 2lb loaf tin with baking paper or use a loaf liner.
Put the chopped chocolate and butter in a large bowl. Microwave for 30 second bursts, stirring, until melted. Stir with a hand whisk until smooth and set aside.
Blitz the bananas, sugar, milk, eggs and vanilla paste in a food processor until smooth or mash the bananas with a fork and stir in the other ingredients.
Stir the purée into the chocolate until smooth.
Sift the flour, cocoa powder and salt over the bowl and then stir it in with a whisk, mixing gently until you have a glossy batter.
Transfer the batter into the prepared tin and bake for 55 minutes – 1 hour or until a skewer inserted in centre comes out clean and cake is risen and firm on top. Cool on a wire rack.
Make the chocolate sauce
Put the cream, sugar, butter and vanilla into a small saucepan. Stir over medium-low heat until the sugar has dissolved and butter has melted. Bring almost to the boil and remove from heat.
Add the salt and chopped chocolate and leave to stand for a couple of minutes. Use a whisk to gently stir until the chocolate has melted completely and the sauce is smooth and glossy.
Leave the chocolate sauce to cool slightly so that it thickens.
Pour the sauce over the cooled cake and decorate with chocolate curls or chopped nuts.
Notes
Keep the cake in a covered container at room temperature for 2-3 days.