Tear the bread into a bowl, cover with the milk, place a weight on top and leave to soak for 15 minutes, until the bread is very soft.
Meanwhile, soak the raisins in grappa, rum or brandy, if using.
Mash or blend the bread and milk mix to a puree.
Add the beaten eggs, cacao powder and brown sugar, then pulse-blend or mix well to combine - the mixture should be the consistency of custard.
Stir in the nuts (save a few tablespoons to scatter on top) and the optional soaked raisins.
Line a 23 cm flan or sealed cake tin with unbleached baking paper and fill with the custard (it's runny, so the tin needs to be a solid one, rather than spring-form).
Bake in a190C (170C fan)/375F/gas mark 5 oven for 40 minutes, until firmly set.
Remove and leave to cool fully in the tin before serving, or refrigerate overnight.
It keeps well for three to five days in the fridge.
