- Preheat the oven to 350 °F (180 °C) and line a muffin tin with paper liners.
- For the hazelnut ribbon, stir the hazelnut butter, brown sugar and vanilla in a small saucepan over medium heat, stirring until the sugar has dissolved. Remove from the heat to cool while making the muffin batter.
- In a large mixing bowl, whisk the banana, sugar and melted butter and then whisk in the eggs and vanilla.
- In a separate bowl, sift the flour, cocoa, baking powder, baking soda and salt together. Add this to the banana mixture and stir until blended. Stir in the chocolate chips. Spoon enough batter to fill each muffin cup two-thirds of the way. Spoon the cooled hazelnut mixture (recipe below) into each muffin cup and then top with the remaining muffin batter (don’t worry if some of the hazelnut paste is still exposed). Bake the muffins for 20-25 minutes, until a tester inserted in the centre of a muffin comes out clean. Cool the muffins in the tin for 10 minutes and then tip out to cool completely.
The muffins will keep in an airtight container for up to 3 days.