Put the hazelnuts in a food processor – a small one is best for this job – and blitz with the metal blade for 8-10 minutes, or until the nuts first turn to a powder and then a glossy paste. Stop the machine and push the nuts down every 3-4 minutes until the correct consistency is reached. The hazelnut paste should look like a runny peanut butter.
Scrape the hazelnut paste into a small saucepan. Finely grate the cacao from the block onto a board. It’s easier to grate and then weigh the cacao rather than cut it off the block at the beginning.
Tip the grated cacao into the pan with the nut paste and warm through very gently for just a few seconds, stirring constantly until the cacao melts. Transfer to a small bowl and serve.
Put the spread in a lidded jar or container. The spread will keep in the fridge, covered, for three days.