Ground hazelnuts are difficult to come by in the shops. You can make your own by blitzing them in a food processor or a coffee grinder. Go steady as it is easy to overdo it and end up with a clumpy, moist mass of ground nuts. If your processor has a pulse setting, then pulse for a second, then stop, assess the progress and pulse again if necessary. I had the best results when I processed the nuts in three batches of 25g in my coffee grinder. Aim for the consistency of ground almonds.