Preheat oven to 350º F (175º C) and grease and flour a bundt pan.
Sift flour, baking soda, baking powder, and salt into bowl and set aside. In large mixing bowl, cream together butter and both sugars for a few minutes until light and fluffy, then add eggs and continue to mix.
Next, add mashed bananas, yogurt and buttermilk and mix well.
Add dry ingredients, and stir until combined.
Remove about 2 cups (235 grams) of the mixture and place in a small bowl. Stir in the chocolate hazelnut spread until evenly mixed.
Place about one third of the white batter into the prepared bundt pan, then top with the entire bowl of the chocolate mixture, and finish putting the rest of the white batter on top. (Optional: place some crushed hazelnuts in the bottom of the pan, if desired.)
With a knife, make a swirling pattern into the batters. I went around twice.
Place into prepared oven, and bake for 50 to 60 minutes or until tester comes out clean.
After about 10 minutes, remove cake from pan and place on cooling rack.
Meanwhile, place confectioner's sugar and coffee into a small bowl and mix together until a thick, but runny consistency is reached, which will drizzle down the cake, without running off completely.
Drizzle icing onto cake when completely cooled.
Let icing set, then serve this lovely banana Nutella swirl bundt cake and accept all the compliments with graciousness.