Preheat oven to 400 F (204 C). Grease and flour a 12-cup regular muffin tin (or a 6-cup jumbo muffin tin).
Using a mesh strainer, sift flour, baking powder, baking soda, cinnamon and salt into a large bowl. Set aside.
2 cups all-purpose flour, 1½ teaspoon baking powder, ½ teaspoon baking soda, ½ teaspoon ground cinnamon, ¼ teaspoon salt
Add the egg, brown sugar, buttermilk, melted butter, and vanilla extract to a blender and blend for about 30 seconds.
1 large egg, ⅔ cup brown sugar, ¾ cup buttermilk, ¼ cup unsalted butter, ½ teaspoon vanilla extract
Pour wet ingredients into dry and stir to combine. Stir until just combined and no flour streaks remain. The batter will be thick.
Fill muffin cups ¼ full with batter.
Add ½ to 1 teaspoon of Nutella to each cup.
1 cup Nutella
Top each cup with more batter until cups are ¾ full.
Bake for 10 minutes then lower oven temperature to 375 F (190 C) and continue baking for 15-20 minutes longer.
Remove to a wire cooling and allow to cool before removing muffins from the pan.