Preheat the oven to 325 degrees F (162 degrees C), with a rack in the middle. Grease and flour a 9-by-5-inch loaf pan.
In a large bowl, whisk the butter and sugar to combine. Add the eggs one at a time, whisking well to combine. Beat in the vanilla and yogurt. Stir in the bananas and mix to combine, using a spatula to smoosh up any big chunks.
In a medium bowl, whisk together the flour, baking soda, cinnamon, and salt. Stir in the cocoa nibs. Fold into the banana mixture, mixing just until fully combined.
Pour half the batter into the prepared pan and top with half the Nutella. Use a small wooden skewer or the blade of a paring knife to swirl the Nutella into the batter. Pour in the remaining batter and finish with the last of the Nutella, swirling again.
Bake until the top of the loaf is golden brown, the sides pull slightly away from the edges of the pan, and a toothpick inserted into the center comes out with just a few moist crumbs attached (see Pro Tips), 75 to 90 minutes; check after 40 minutes, and if the cake is browning too quickly, tent loosely with aluminum foil. Cool the loaf in the pan for 15 minutes, then turn out onto a wire rack to cool completely.