Start the day before blending the milk. Preheat the oven to 180C/350F/gas mark 4. Spread the hazelnuts out on a baking tray and toast in the oven for 10-15 minutes until lightly browned and fragrant.
Tip the nuts into a clean tea towel, wrap them up and allow to sit for a minute. Rub the hazelnuts in the tea towel to remove the skins; don't worry if you don't manage to get them all off.
Place the skinned hazelnuts in a bowl and cover with cold water. Place in the fridge and leave to soak overnight, or for 8 hours.
Drain the soaked nuts and place in a blender with the coffee, cocoa powder, 4 Tbsp of the maple syrup and the vanilla extract. Blend until very smooth.
Pour the mixture into a nut milk bag or clean muslin cloth/tea towel set over a large bowl or jug. Squeeze the milk through the cloth to remove any solids; make sure that you squeeze as much liquid as possible through for the best flavour.
Discard the solids (or keep to use in granola/cookies). Taste the milk and add more maple syrup as needed. Place in the fridge to chill before serving.
Serve over ice. Milk will keep in the fridge for up to five days.
If you want to skip the step of skinning the hazelnuts then you can buy blanched hazelnuts rather than the kind with skin. I still recommend toasting them in the oven until golden however as this will give the best flavour.