Sterilize a 400 g (13 oz) glass jar (see
Grind the hazelnuts into a paste in a food processor. Add the oil, sugar, cocoa powder and vanilla extract and whiz again, to combine.
Pour in the melted chocolate and blend until the mixture is very smooth, warm and runny. Pour it into the sterilized jar and cover. Leave to cool and thicken slightly before using. It will keep for up to 1 month, covered, at room temperature.