Melt the chocolate in a microwave oven in 30-second bursts, stirring it between each until it's completely melted, smooth, and pourable. (This can also be done in a bowl over a pan of boiling water.)
Pour the chocolate into the pie shell. With a spoon, spread it around to coat the inside of the pie shell evenly.
Combine all filling ingredients except the butter in a metal bowl set over (but not touching) simmering water. Whisk briskly until the mixture thickens and becomes fluffy--about 10-15 minutes.
When the mixture has reached the desired thickness, whisk in the butter.
Pour the mixture into the pie shell. Put the pie into the refrigerator to set for about two hours.
Decorate pie with shaved chocolate, whipped cream, or lime slices.
