Preheat the oven to 350°F (175°C). Line a 12-cup muffin tin with cupcake liners.
In a medium bowl, whisk together the all-purpose flour, baking soda, salt, ground cinnamon, and ground nutmeg until well combined.
In a large bowl, mix the melted butter and granulated sugar until smooth.
Add the eggs one at a time to the butter and sugar mixture, beating well after each addition.
Stir in the vanilla extract.
Add the mashed bananas and sour cream to the wet ingredients and mix until combined.
Gradually add the dry ingredients to the wet ingredients, stirring gently until just combined. Do not overmix.
Fold in the chopped walnuts if using.
Divide the batter evenly among the 12 cupcake liners, filling each about 3/4 full.
Bake for 18 to 22 minutes, or until a toothpick inserted into the center of a cupcake comes out clean.
Remove the cupcakes from the oven and let them cool in the pan for 5 minutes.
Transfer the cupcakes to a wire rack to cool completely before serving.