Preheat the oven to 350°F (175°C) and line cupcake pan with paper liners.

Sift together all-purpose flour, granulated sugar, cocoa powder, baking powder, baking soda, and salt in a large bowl.

In a separate bowl, whisk together egg, milk, vegetable oil, and vanilla extract until smooth.

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

Gradually add hot coffee or water while stirring until the batter is silky and slightly runny.

Fill each cupcake liner halfway with batter and bake for about 18 minutes until tops spring back when pressed.

Let the cupcakes cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.

Make the Nutella buttercream by beating butter until creamy, then adding sifted powdered sugar, cocoa powder, Nutella, and heavy cream until fluffy.

Core each cooled cupcake and fill with Nutella using a piping bag or spoon.

Pipe Nutella buttercream onto each filled cupcake and optionally decorate with chopped hazelnuts.