Preheat 190C/375F/gas 5. Grease/line a baking tray. Blitz butter, xylitol and rice syrup in a food processor until light and fluffy. Mix in the flour and ground hazelnuts and pulse to combine, adding a little egg yolk if needed to make a soft, slightly sticky dough.

Roll about 1 tsp of mixture into a ball, then place on the baking tray and push down slightly, making a small indent in the middle with your finger. Place a whole hazelnut in the centre of the cookie and push it in ever so slightly. Repeat with the remaining mixture, then bake for about 15 mins, or until slightly browned and firm. Leave to cool completely before serving. Move them delicately if transferring to a cooling rack, they are very soft and crumbly! If using, drizzle over some melted chocolate and/or sprinkle over a little coconut flour or icing sugar.