Prepare the Cupcakes: Preheat oven to 350°F (175°C). Line a 12-cup muffin tin with cupcake liners.
Mix the Dry Ingredients: In a bowl, whisk together flour, cocoa powder, baking powder, baking soda, and salt.
Mix the Wet Ingredients: In a separate large bowl, beat the eggs with granulated and brown sugar until light and fluffy. Add oil, buttermilk, vanilla, and Nutella, mixing until smooth.
Combine Batter: Gradually add the dry ingredients into the wet, stirring until just combined.
Bake: Divide the batter evenly among cupcake liners. Bake for 16–18 minutes, or until a toothpick inserted comes out clean. Let cool completely.
Optional Filling: Using a knife or cupcake corer, remove a small portion from the center of each cupcake. Fill with a small spoonful of Nutella.
Make the Frosting: Beat butter until light and creamy. Add Nutella and beat to combine. Add powdered sugar, cream, vanilla, and salt. Beat until smooth and fluffy.
Frost and Garnish: Pipe or spread frosting onto cooled cupcakes. Sprinkle with chopped toasted hazelnuts.