Preheat the oven to 180°C conventional oven (356° Fahrenheit/Gas 4). Line the base and sides of a 19 or 20cm round, loose-bottom cake pan with butter, then place baking paper over the base and sides of the cake pan. If using wafer paper, cut it to be the same size as the base and place inside the cake pan. Set aside the prepared cake pan with wafer paper to line the base.
To make the Panforte, place the honey, water and caster sugar in a medium-sized saucepan. Bring to the boil over low heat (approximately 5 minutes). Remove saucepan from the heat. Use a wooden spoon to stir the candied peel into the honey and sugar liquid. Stir in the cinnamon, cloves and nutmeg, then slowly stir in the flour until well combined. Stir in the almonds and hazelnuts.
Spoon the Panforte mixture into the prepared cake pan on top of the baking paper (and wafer paper if using). Use the back of a spoon to flatten the Panforte to an even surface. Cover with aluminium foil and bake for 25 minutes. Set aside to cool before serving. Dust with icing sugar before cutting into wedges to serve.
Note: Panforte will keep for about 1 week at cool room temperature or in the fridge.