Melt the butter over low heat and add the mace and cayenne. Let this steep gently for 10 minutes, then add the shrimp and mix well. Cover. If the shrimp are pre-cooked, turn off the heat. If not, cook on the lowest setting until they all turn pink. Mix in the lemon juice.
Serve over good bread or crackers, with black pepper on top.
Potted shrimp are better the day after they're made, so if you want to do that, pack them into a jar, then pour the butter over them and refrigerate. If the butter doesn't cover the shrimp, it's OK if you're going to eat them all in a day or three. If you want to store them for up to a month, you need to make sure the shrimp are totally covered by the butter.