Preheat the oven to 350˚F (177˚C). Line a muffin pan with 12 paper liners.
In a large bowl, whisk together flour, baking soda and salt.
In another large bowl, whisk together oil and sugar. Add eggs and sour cream, and whisk until just combined. Add mashed banana and dry ingredients. With a wooden spoon or rubber spatula, gently fold everything together until just combined.
Fill each paper liners half full with batter. Scoop about ¼ teaspoon Nutella and add to each muffin cup in a few additions, so the Nutella is spread out (not in a dollop). Fill the muffin cups with more batter until ¾ full. Once again, scoop about ¼ teaspoon Nutella and add to each muffin cup in a few additions. Using a toothpick, slightly swirl the top part of the muffin.
Bake in the oven for 17 – 20 minutes or until golden brown on top and the middle springs back when press slightly. Remove from oven and cool in the muffin pan for 5 minutes. Serve or transfer the muffins to a cooling rack and cool completely.