Preheat the oven to 220°C /430F/Gas Mark 7. Line a 12 cup muffin tin with cupcake liners.
Roughly chop the walnuts and chocolate bar (if not using chips), and set them aside. Mash the bananas using a fork or potato masher and set them aside.
70 grams walnuts, 150 grams milk chocolate chips, 2 large ripe bananas
Whisk together the flour, baking soda, and salt. Set aside.
300 grams plain flour, 1½ teaspoons baking soda, ½ teaspoon salt
Whisk together the softened butter, eggs, yoghurt, sugar and vanilla extract.
150 grams soft brown sugar, 2 large eggs, 60 grams butter, 90 grams Greek-style yoghurt, 3 teaspoons vanilla
Add in the flour mixture, walnuts, chocolate chips and mashed bananas. Fold together until just mixed.
Divide the batter into the 12 cupcake liners. Top with a teaspoon of cinnamon sugar (optional extra), and bake at 220°C /430F/Gas Mark 7 for 5 minutes. Turn down the oven to 200°C/390F/Gas Mark 6 and bake for a further 12 to 15 minutes, until just baked and a toothpick or cake tester comes out mostly clean.
Allow the muffins to cool on a cooling rack before serving.