Preheat the oven to 350°F. Grease a loaf pan very well with butter (or line with parchment paper) and set aside. Combine the eggs, sugar, and vanilla in a large mixing bowl. Whisk to combine. Add the butter and sour cream and whisk again.
Add the flour, baking soda, and salt to the bowl. Whisk to combine. Add the mashed banana and stir again. Add the chocolate chips and the walnuts and stir once more to mix throughout.
Pour the batter into the prepared loaf pan. Bake for 60-65 minutes until golden brown and a toothpick inserted comes out clean. Cool in the pan for 10 minutes before removing from the pan. Let cool completely on a wire rack before slicing.