In a the bowl of a stand mixer fitted with the paddle attachment, cream together butter and sugar. Add the two very ripe bananas and eggs and mix until banana has been smashed up. Add sour cream and mix until combined.

In a separate bowl, mix together flour, baking powder, salt and chocolate chunks and toss to combined. Add dry ingredients to wet and mix until the flour is incorporated (do not overmix!).

Preheat your oven to 325 degrees F. Grease a 9x5" loaf pan. Fill pan 1/3 full of batter. Dot top of batter liberally with scoops of nutella and spread slightly to evenly cover top of batter. Cover nutella with batter until pan is 2/3 full. Take the remaining banana and slice in half lengthwise. Press banana, cut side up, into the batter. Dot the top with more nutella. Sprinkle banana slices with turbinado sugar. Bake until a cake tester comes out clean, about 60-75 minutes depending on your oven. Start checking bread after about 50 minutes. Cool slightly before removing from pan and slicing.