Preheat oven to 400ºF. Spray 15 wells of two 12-count muffin pans with non-stick spray or line the wells with cupcake liners. Set aside.
In a large bowl, toss together the flour, baking powder, and salt. Set aside.
In a medium size bowl, whisk together the butter and granulated sugar until smooth. Add the eggs, buttermilk, and vanilla extract and whisk again until everything is combined.
Pour the wet ingredients into the dry ingredients and stir gently with a spatula until just combined.
Spoon 2 Tablespoons of the batter evenly into the prepared wells. Top each mound of batter with 1 teaspoon of Nutella. Using a toothpick, swirl the Nutella into the batter. Top with another 2 Tablespoons of batter and an additional teaspoon of Nutella. Swirl again. Bake muffins for 5 minutes, then, without opening the oven door, reduce heat to 350ºF and bake for an additional 12-13 minutes until a toothpick inserted in the center comes out mostly clean. Remove from oven and allow to cool completely on a wire rack. Muffins will stay fresh in an airtight container at room temperature up to 5 days. Muffins freeze well, up to 2 months. Thaw in microwave for 45-60 seconds before eating.
It is always a good rule of thumb to use room temperature eggs when using room temperature butter. To bring eggs to room temperature quickly, place in a bowl of warm water for 10 minutes.
If you don't have buttermilk, you can easily make your own: see my post about homemade buttermilk.