Preheat oven to 350°F and line a cupcake pan with cupcake liners. Set aside.
In a small bowl, stir together flour, cocoa powder, baking powder, and salt. Set aside.
In a large mixing bowl, whisk together oil, granulated sugar, brown sugar, and egg until smooth.
Alternate adding in half of the flour mixture and then half of the milk, whisking after each addition. Repeat with the remaining flour mixture and milk and whisk just until combined and no big lumps remain.
Fill cupcake liners about ¾ full with the batter.
Bake cupcakes for about 30-35 minutes or until a toothpick inserted in the middle comes out clean. Let cupcakes cool in the pan for about 15 minutes before transferring them to a wire rack to cool completely.
Using the end of a medium sized piping tip, twist it into the middle of each cupcake about ⅓ deep to cut out a small hole. Fill each cupcake with Nutella or hazelnut spread.
Chocolate Buttercream Frosting
In the mixing bowl of a stand mixer fitted with a paddle attachment, beat softened butter on medium-high speed for 10 minutes until very creamy and pale in colour.
Add in the powdered sugar and beat on low speed, increasing to high speed for 5 minutes.
Scrap down the bowl and add in melted dark chocolate. Continue beating for another 5-10 minutes until fluffy and creamy. Finish by smearing it around the bowl with a spatula to get rid of some air bubbles.
Transfer buttercream to a piping bag fitted with a piping tip of your choice. Pipe your design on top of each cupcake. Press a Ferrero Rocher chocolate on top of each cupcake and sprinkle buttercream with crushed hazelnuts.