Preheat the oven to 180°C, gas mark 4. For the tart crust, tip the hazelnuts onto a roasting tray and roast for 6-8 minutes until golden; set aside to cool completely. Once cool tip the nuts into the bowl of a food processor and whizz, pulsing until just finely ground, (take care not to over-blend or the nuts will release too much oil). Tip in the cacao and pulse again until combined, then add the maple syrup, coconut oil and a pinch of sea salt. Pulse until just combined.