Melt chocolate and cream in a double boiler or a microwave. Stir in cayenne.
Cover and chill at least 3 hours.
Mix powders in a bowl. Scoop out some chocolate; mold it into a ball;
roll it in the coating; repeat.
Alternately, chill colder and cut the chocolate into cubes before coating it.
Store at room temperature.
The point of truffles is to melt in the mouth.
This requires storage at room temperature and may require seasonal adjustment of the level of cream. I use 1/4 C cream in the summer. When it is very hot, I do refrigerate. Reducing the cream may require reducing the pepper.
I had several terrible experiences using butter to make truffles and swore it off for a decade, but I have am experimenting with it now. Keep in mind that cream is basically equal parts butter and water.