Melt the butter in a heavy, large non-stick pan over medium heat. Add coconut sugar and walnuts, stirring constantly until the sugar is melted and nuts are evenly coated. The sugar will become sandy and then begin to caramelise and soften; at this stage, remove from the heat and pour the nuts onto a parchment-lined baking sheet, spreading the walnuts around so they don’t stick together. Allow to cool completely and serve.