Preheat oven to 350 degrees. Spray 8×4-inch loaf pan with nonstick spray. In a bowl, stir together flour, baking soda, and salt. In a large bowl, beat sugar and butter until light and fluffy. Add eggs one at a time, beating well after each addition. Add banana, milk, and vanilla until combined. Gradually add flour mixture, beating constantly until just blended. Spoon Nutella into a small dish and soften in the microwave for about 15 seconds. Add 1 cup of the banana bread batter to the Nutella and stir until blended. Add half the banana bread batter to the prepared pan. Top with half the Nutella batter. Pour the remaining banana batter into the pan and top with the remaining Nutella. Swirl batters together with a knife. Bake for 50-55 minutes (the bread will seem undercooked). Cool for 10 minutes, then turn out onto a wire rack to cool completely.