Preferably the night before, roast hazelnuts in a single layer on a baking tray at 180C/350F for about 10 minutes until toasted and aromatic and the skins turn a couple of shades darker.
Remove from oven and while the hazelnuts are still warm, rub between a towel or paper towel or your fingers to remove the skins. Aim to remove at least 70% of the skins (which can be quite bitter). If there are some stubborn ones, you can pop them back in the oven for a few more minutes and try again.
Let the hazelnuts cool completely – preferably overnight – or they will not blend as finely as they should (and will be harder work for your blender).