Preheat the oven to 350F degrees. Line a standard sized muffin pan with cupcake liners. Set aside.
In a large bowl, whisk the cocoa powder, flour, baking soda, baking powder, and salt together until well mixed. Set aside.
In a medium bowl, whisk the eggs, sugar, brown sugar, oil, and vanilla together until completely smooth.
Pour half of the wet ingredients into the dry ingredients. Pour in half of the buttermilk* and gently whisk for a few seconds. Repeat with the remaining wet ingredients and buttermilk. Stir until just combined.
Spoon the batter into the liners until they are halfway filled.
Bake for 18-19 minutes or until a toothpick inserted in the center comes out clean. Allow to cool completely before frosting.
*If you don't have buttermilk, you can make your own: mix 1 teaspoon white vinegar or lemon juice with ½ cup milk. Stir and let sit for 5 minutes.
For the Nutella Frosting:
In a large bowl, beat the butter until it is light and fluffy. Gradually add the powdered sugar and salt until smooth Add vanilla and beat until combined. Add Nutella and beat until light and fluffy.