Heat oven to 375°F. Line a 12-cup muffin pan with paper liners.
Whisk flour, baking powder, baking soda, salt, and cinnamon in a bowl.
Mash bananas in a large bowl. Whisk in granulated sugar, brown sugar, oil, buttermilk, and vanilla until smooth.
Fold dry ingredients into the banana mixture just until no dry streaks remain.
Stir in chopped hazelnuts, keeping the batter slightly lumpy.
Microwave the chocolate-hazelnut spread for 10–15 seconds to loosen.
Spoon batter to fill cups halfway. Add about 2 teaspoons of spread to each, then top with more batter.
Swirl the tops gently with a toothpick for a marbled look.
Bake 16–18 minutes, until a tester comes out clean at the edge of a muffin.
Cool in pan 5 minutes, then move muffins to a rack to finish cooling.