Preheat & Prep: Preheat oven to 350°F. Line a muffin tin with paper liners. Ensure all ingredients are at room temperature.
Mix Dry Ingredients: In a large bowl, whisk together flour, sugar, cocoa powder, baking powder, baking soda, and salt.
Mix Wet Ingredients: In a separate bowl, whisk eggs, buttermilk, oil, and vanilla extract until smooth. Warm Nutella in the microwave for 20 seconds, then mix into the wet ingredients.
Combine & Make Batter: Gradually mix wet ingredients into dry until just combined. Stir in hot coffee last; batter will be thin.
Bake Cupcakes: Fill liners ⅔ full (about 3 tbsp of batter). Bake for 17-19 minutes or until a toothpick inserted comes out with a few crumbs.
Cool Completely: Let cupcakes cool in the pan for 5 minutes, then transfer to a wire rack.
Make Frosting: Beat butter for 3 minutes until fluffy. Add Nutella and beat for 2 more minutes. Gradually add powdered sugar, heavy cream, extract, and salt. Beat on high for 3-4 minutes.
Decorate: Pipe frosting onto cooled cupcakes. Place a Ferrero Rocher on top, then sprinkle with chopped hazelnuts, chocolate sprinkles, and gold sprinkles.