Heat half the butter in a saucepan over moderate heat, skimming off and discarding the foam as it rises to the top.
Add the shrimp, salt, pepper, mace, and optional cayenne and heat without boiling.
Spoon the mixture into 4 ramekins.
Melt the remaining butter, skimming off and discarding the foam as it rises to the top, and spoon over the shrimp, leaving the sediment behind.
Cool until the butter is almost set and place a small sprig of dill on the surface of the butter.
Chill in the refrigerator and let sit at room temperature for at least 30 minutes before serving.
Serve with bread or toast points and lemon wedges.