The old-favourite vanilla slice gets a twist here with the addition of Nutella and coffee and chocolate in the icing. Getting the icing right is important - too thick and it won't spread; too thin and it will run off. You can eat this freshly sliced, but it's quite sticky; best to let it set in the fridge first if you have time. Start this recipe a day ahead to make the hazelnut custard.
This recipe makes about 10 pieces.