Preheat the oven to 325 degrees F. Generously coat an 8 1/2-inch-by-4 1/2-inch loaf pan with nonstick spray and dust with flour, tap out the excess.
Whisk together the flour, baking soda and salt in a medium bowl and set aside. Mash the sugar with the bananas in a large bowl until smooth. Whisk in the butter, vanilla and eggs. Whisk in the flour mixture until just combined.
Microwave the chocolate-hazelnut spread in a small microwave-safe bowl and until pourable, about 30 seconds. Reserve 2 tablespoons in a small bowl and set aside. Spread half of the batter in the bottom of the prepared pan. Pour the warmed hazelnut spread down the center of the batter leaving about a 1/2-inch border all around so that you have a chocolate-hazelnut center. Spread the remaining batter on top and smooth out with a spatula.
Bake until the top is domed, about 1 hour 15 minutes. Cool in the pan and on a wire rack for 30 minutes, then carefully invert onto a platter or cake stand and then turn upright. Cool completely on the wire rack, about 1 hour. Microwave the reserved hazelnut spread until it can be drizzled, about 30 seconds. Drizzle the spread over the cake then sprinkle the hazelnuts down the center.
When measuring flour, we spoon into a dry measuring cup and level off the excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)
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