Preheat the oven to 350°F (175°C) and grease a loaf pan.

In a mixing bowl, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy.

Add the mashed bananas, sour cream, beaten egg, and vanilla extract. Mix until well combined.

In another bowl, whisk together the flour, baking soda, and salt.

Gradually add the dry ingredients to the wet ingredients, stirring until just combined. Do not overmix.

Pour the batter into the prepared loaf pan and smooth the top.

Bake in the preheated oven for 60 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.