Combine crab, cayenne, lemon juice and zest, and season well. Pack into 8 small or 1 large ramekin. Melt the butter, then pour over until the crab is covered with a thin film. Top with a parsley leaf if you like, then cool in the fridge. (It will keep for up to 2 days in the fridge.) To prevent the butter melting, transport to your picnic in a chiller bag.