Preheat your oven to 350°F (175°C) and grease a 9×5 inch loaf pan.
In a medium bowl, whisk together the flour, baking soda, and salt.
In a large bowl, cream together the butter and sugar until light and fluffy. Add the eggs one at a time, beating well after each addition, then mix in the vanilla extract.
Stir in the mashed bananas and sour cream until well combined.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Fold in the chopped walnuts and cinnamon.
Pour the batter into the prepared loaf pan and smooth the top.
Bake for 60-70 minutes, or until a toothpick inserted into the center comes out clean. Allow the bread to cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.
To make the frosting, beat the butter until creamy. Gradually add the powdered sugar, espresso powder, heavy cream, and vanilla extract, beating until smooth and spreadable.
Once the bread has cooled, spread the espresso frosting over the top. Slice and enjoy!