1 Preheat the oven to 350°F (175°C). Grease a 9×5-inch (23×13 cm) loaf pan and line with parchment paper, leaving an overhang for easy removal.

2 In a bowl, mash the ripe bananas with a fork until mostly smooth with a few small lumps.

3 In a large mixing bowl whisk together the melted butter and brown sugar until combined. Add the egg, vanilla extract and sour cream (or yogurt) and whisk until smooth.

4 Stir the mashed bananas into the wet mixture until evenly incorporated.

5 In a separate bowl, sift or whisk together the flour, baking soda, salt and ground cinnamon.

6 Fold the dry ingredients into the wet mixture gently, mixing just until combined — avoid overmixing to keep the bread tender.

7 Fold in 3/4 of the chopped toasted walnuts, reserving the rest for the top.

8 Pour the batter into the prepared loaf pan and smooth the top. Sprinkle the reserved walnuts and optional coarse sugar over the batter.

9 Bake for 50–60 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs (not wet batter). If the top browns too quickly, tent loosely with foil for the last 15 minutes.

10 Let the loaf cool in the pan for 10–15 minutes, then lift out using the parchment overhang and transfer to a wire rack to cool completely before slicing.

11 Slice and serve slightly warm or at room temperature. Store wrapped at room temperature for up to 3 days or freeze slices for longer storage.