Place the finely chopped chocolate in a heatproof bowl.
In a small saucepan, bring the cream to the boil and pour it over the chocolate.
Set aside for 2 - 3 minutes, then stir, starting from the middle, until the chocolate is melted and incorporated with the cream.
Set aside in the refrigerator until the ganache firms up.
Add the butter and Nutella to the ganache and use a stick blender to blitz the mixture until it is light and fluffy (you can also do this in a food processor).
Match the macaron shells up so they have partners of similar size. Upturn one shell of each pair of macarons.
Spoon the ganache into a piping bag fitted with a medium round nozzle.
Pipe a little ganache onto the center of each upturned macaron. Sandwich it together with its partner.
Keep refrigerated. Makes 50 - 60 macarons.