Preheat the oven to 350°F (175°C) and prepare a cupcake tin with liners or grease.

In a large mixing bowl, sift together the dry ingredients: flour, sugar, cocoa powder, baking powder, baking soda, and salt.

In another bowl, whisk together the wet ingredients: egg, milk, oil, and vanilla until fully combined.

Pour the wet mixture into the dry ingredients and fold together until just combined.

Slowly stir in brewed coffee or hot water until the batter reaches a syrup-like consistency.

Fill the cupcake liners halfway with batter and bake for about 18 minutes.

Let the cupcakes cool in the tin for 5 minutes before transferring them to a wire rack.

For the frosting, beat the softened butter until creamy, then add powdered sugar, Nutella, cocoa powder, and heavy cream, mixing until smooth.

Once cooled, remove the center of each cupcake and fill with a dollop of Nutella.

Frost the tops of the filled cupcakes with Nutella buttercream and sprinkle with chopped hazelnuts if desired.