Step 1: Preheat your oven to 350°F (175°C) and line a 12-cup muffin tin with paper liners.
Step 2: In a large bowl, sift together the flour, cocoa powder, sugar, baking powder, baking soda, and salt.
Step 3: In a medium bowl, whisk together the vegetable oil, egg, buttermilk, and warm water until combined.
Step 4: Gradually pour the wet ingredients into the dry ingredients, whisking until the batter is smooth. Fold in the chopped hazelnuts.
Step 5: Divide the batter evenly among the cupcake liners and bake for 18-20 minutes, or until a toothpick comes out clean.
Step 6: Let the cupcakes cool completely on a wire rack. Once cool, use a small knife to remove a small core from the center of each cupcake.
Step 7: Fill each cavity with one teaspoon of Nutella and replace the top portion of the cake core.
Step 8: For the frosting, beat the softened butter until light and fluffy. Add the powdered sugar and Nutella, and beat for another 3-5 minutes until silky.
Step 9: Pipe the frosting onto the cupcakes and top each with a whole Ferrero Rocher truffle.