Melt the chocolate in the microwave in 30-second intervals. Allow to cool to room temperature.
Put the Gentle Sweet (or other sugar alternative) in a blender and blend until it becomes a very fine powder. Reserving 1 tbsp, pour the rest into a large bowl. Add the butter, and with an electric mixer, cream the Gentle Sweet and butter for about 2 minutes.
Once the melted chocolate is close to room temperature (you don't want it to melt the butter), add it to the bowl along with the 1/4 C heavy cream, and beat for another 2 to 3 minutes. Beat in the vanilla and salt. Beat in the egg whites, 1/4 C at a time, beating well, about 2 minutes after each addition. Then beat the mixture for at least another couple of minutes, or until the mixture is nicely thickened.
Pour the 2/3 C heavy cream into the blender with the reserved Gentle Sweet and blend until thickened. (Or you can make whipped cream with your stand or hand mixer)
Scrape the chocolate mixture into the baked pie crust, top with the whipped cream and optional shaved chocolate. Chill for a few hours.