Preheat your oven to 350°F (176°C) and line a cupcake pan with cupcake liners.

In a medium-sized bowl, combine the flour, baking powder, baking soda, and salt, and set aside.

In a large bowl, whisk together the melted butter, sugar, and vanilla extract until well combined.

Add the eggs and whisk until fully incorporated.

Add the sour cream and Nutella, whisking until well combined.

Now, add the milk and whisk until fully combined.

Gradually add the dry ingredients and whisk until just combined, being careful not to overmix.

Fill the cupcake liners a little more than halfway full (about 51g of batter per liner).

Bake for 16-20 minutes, or until a toothpick inserted into the center comes out clean and the edges are lightly golden.

Remove the cupcakes from the oven and let them cool on a wire rack.

While the cupcakes are cooling, prepare the frosting. In a large mixing bowl, combine the butter and Nutella, and beat until smooth and well combined.

Add about half of the powdered sugar and mix until smooth and fully incorporated.

Add 2 tablespoons of heavy cream and mix until well combined.

Add the remaining powdered sugar, mixing until smooth.

Add the remaining cream as needed, continuing to mix until the frosting reaches a smooth consistency.

Once the cupcakes are completely cool, use a cupcake corer to remove the centers.

Fill each cupcake with the additional Nutella.

Pipe the buttercream on top of each cupcake using Ateco tip 847.

Finish each cupcake with half of a Ferrero Rocher candy, if desired, and optionally sprinkle with finely chopped hazelnuts for added texture.