Preheat oven to 425°. In a small mixing bowl melt the butter. Allow to cool for a few minutes.
Make the streusel topping. Combine the softened butter, flour, brown sugar, sugar and cinnamon in a small bowl. Mix with a fork until crumbly and well combined. Save for the last step before baking.
Line your muffin tin (12 cups) with muffin cups.
In a large mixing bowl, combine the flour, sugar, baking powder, baking soda and salt. Whisk to combine.
In a large measuring cup, combine the cooled butter, sour cream, buttermilk, eggs, and vanilla. Whisk until well combined.
Add the wet ingredients to the dry ingredients. Mix until just combined. Be sure not to overmix.
Gently fold in the hazelnuts and chocolate chips.
Use a large cookie scoop to add batter to the muffin pan. Fill each muffin cup half way.
Add a teaspoon of Nutella to the top of the muffin batter. Top muffins with the remaining batter.
Place a spoonful of streusel topping on the top of each muffin. Press down just a little bit so that the streusel sticks to the batter.
Bake muffins at 425° for 5 minutes. Lower the temperature to 375° and bake for an additional 16-18 minutes or until the edges are golden brown and the middles are baked through.
Remove muffins from the oven and allow the muffins to cool. Once muffins are cool, drizzle with a little more Nutella before serving.
Notes
Bake at a Higher Temperature for the First Five Minutes: To guarantee big, fluffy, bakery-style muffins, bake the muffins for five minutes at a higher temperature - 425°. Then lower the temperature for the rest of the baking time to 375°.Work Quickly: Even though you have frozen the berries, they will quickly become soft again. Try to get you batter scooped and into the muffin pan as quickly as you can. This will prevent the blueberries from breaking when you scoop the batter.Substitutes for Buttermilk:Buttermilk Powder - available in your local grocery store or online, simply follow the instructions on the can to make your own buttermilk at home.Milk and Lemon Juice or Vinegar - combine 1.5 teaspoons lemon juice or white vinegar with ½ cup of milk. Whisk until well combined and use in place of buttermilk in the muffin recipe.