To make the macaron shells, start by lining two baking sheets with parchment paper and have a pastry bag with a plain tip (about 1/2-inch, 2 cm) ready. You may also use a Ziploc bag with the tip cut off, for piping.
Grind together the powdered sugar, the powdered almonds and coffee powder so there are no lumps.
With a hand held electric mixer, beat the egg whites until they begin to rise and hold their shape. While whipping, beat in the granulated sugar until very stiff and firm (about 2 minutes).
Carefully fold the dry ingredients, in two batches, into the beaten egg whites with a flexible rubber spatula. When the mixture is just smooth and there are no streaks of egg white, stop folding and scrape the batter into the pastry bag/ Ziploc bag (standing the bag in a tall glass helps).
Pipe the batter on the parchment-lined baking sheets in 1-inch circles (about 1 tablespoon each of batter), evenly spaced one-inch (3 cm) apart.
Rap the baking sheet a few times firmly on the counter top to flatten the macarons. Keep them at room temperature for about an hour till a thin film forms on the macaron batter. If you lightly touch a macron and the batter doesn't stick to your finger, then it is ready.
Bake them at 160C (320F) or 170C (340F) for minutes. Let the macarons cool completely then slowly peel off the parchment paper. Store the macarons in airtight containers till ready to fill.
To make the ganache filling, heat the cream in a small saucepan. When the cream just begins to boil at the edges, remove from heat and add the chopped chocolate. Let sit one minute, then stir until smooth. Add the Nutella and mix well. Let cool completely before using.
To assemble the macarons, spread (or pipe) a bit of ganache on the inside of the macarons then sandwich them together.
You may or may not use all the filling, depending on how much you use in the macarons. Let them stand at least one day before serving, to meld the flavors.Store in an airtight container for up to 5 days.