Sift all of the dry ingredients into a bowl and set aside.
Cream the butter and brown sugar until smooth and creamy.
Mix in the eggs one at a time, making sure each egg is incorporated completely before adding the next egg.
Add vanilla and beat for about 30 seconds.
Alternately add about ⅓ of the dry ingredients and a ⅓ of the buttermilk and mix until smooth and creamy.
Add the 1 cup of Nutella and blend until completely incorporated.
Line muffin pans with papers and using an ice cream scoop fill each liner about ¾ full.
Using a ¼ teaspoon measuring spoon scoop a bit of Nutella and place on top, in the center of the batter of each muffin. Use a tooth pick to lightly marble the Nutella and the muffin batter.
Bake in pre-heated oven for 25-30 minutes, adjusting the time for your particular oven. Muffins should spring back when done.